Hi all! Thank you so much for the amazing comments! I was out of town yesterday, so I'm still trying to catch up, but there are two things I want to address overall right quick. 1. I want to reiterate that as of next month, I will no longer be in possession of this specific drum, it will be in the machine at System Source museum on display. All of the suggestions of companies and people to reach out to are greatly appreciated and I'm making a list, but I won't be reaching out to anyone until I'm sure on a timeline as to when the drum will be back in the shop here. Rest assured, I'm not ignoring your suggestions! 2. I of course would first build a replica of the drum to test things on. Probably a chunk of aluminum with the same diameter, but maybe only 1/4 as wide. This way I can test application methods, machining methods, grinding/polishing methods, etc. without worrying about damaging the original drum. Once we have a workflow hammered out on the test piece, we'll move over to work on the original drum. The goal is to do all of our testing on a sacrificial piece, so that once we get to the original drum, we only have to work on it once.

I'm trying to remember what I knew about magnetic tape. If my 73 year old memory serves me right, the magnetic material in tape was gamma ferris oxide that was laid down on the tape in a solvent and the gamma ferric oxide was oriented using a magnetic field perpendicular to the tape surface. If you have a random orientation of the ferric oxide, then the signal level might not be high enough.

electroplating might be your best solution. When scientists and engineers experimented with tube computers back in the 50es and 60es in Hungary, they used rotating drum memories just like this computer did. When they had head crashes, they stripped the coating off the drum on a very precise lathe, and used electroplating to deposit a new layer. As i read, they did not use ferrous oxide, they used some king of chromium salt, soluble in water. The finish ended up to be very strong and durable, but needed higher magnetizing currents. The article did not tell the exact composition, i guess it was a trial and error thing. In that era, every computer was an unique design, especially in Eastern Europe.

You may want to pick up a paint thickness gauge before you put the recoated drum back in the machine. This will let you test the consistency across the drum surface, so you can rule out high spots causing crashes. These are commonly used by detailers, so they know how much paint they can safely remove when polishing.

I *love* your idea for recoating the surface of the drum. Just a few thoughts: 1. When you machine off the old oxide you may find the drum is a bit oddly shaped. Unless you know for sure that the drum was machined while mounted to the spindle that it's currently on, there may have been some misalignment between the center of the drum and the center of the shaft when the two pieces were brought together. If that happens then you'll need a plan for dealing with that 2. Please be sure to keep the drum rotating at a slow speed while it dries COMPLETELY. You don't want any of the new layer flowing due to gravity after the coating is complete 3. Practice makes perfect, so if you can find a scrap piece of aluminum about the right size that you can machine first then that would be ideal I hope for all the best for you. You are a much better and braver man than I. 🙂

One option you might want to consider is making a stand-in drum memory from scratch - something you can use to refine the process for cleaning/coating/etc without impacting the original while you work out the details. If you can get THAT working (and clean/re-do the coating multiple times), then you will have a fairly solid process before proceeding to the real one. Primary concerns on the mechanical side would be dealing with runout from re-mounting things (so turning between centers, and indicating things in REALLY WELL, including having a perfectly "leveled" lathe). I'm REALLY excited to see this happen, it combines some of the coolest electro-mechanical problems from the era!

The right guy to ask would be Ben from Applied Science here on KZfaq. He may not have any experience with coating drums as such, but he did a lot of processes involving coating and chemistry with similar requirements.

As for the drum rebuild: I would strongly suggest to get the drum CNC-Ground down to H6 or better tolerances with a Ra of ~0.2µm or better and a cylindricity of 0.002mm or better. The better your starting point, the higher the chances of the rebuild working. The coating itself should be done using the correct binder. perhaps you can find an old IBM patent somewhere describing the binders used in drum or disk coatings. Also make sure to properly measure the coating thickness. If you had the money I would suggest getting the coating sputtered on using a modern magnetron-sputtering system. I would definitely advise against using a lathe. Especially if it hasn't been properly trammed and aligned within itself and to gravity using a machinists level.

I used that keypunch when I was an undergrad in the early 70's.

Don’t forget the bearings. A few microns of runout in the bearing could easily result in 1 mil + at the edge of the drum. That’s really a crazy tight tolerance for a machine built in the early to mid 1950s. I am thinking a lacquer with the correct ferrite compound sprayed on is the way to go. I have my doubts about polishing the surface after the fact. I have a feeling how drums and platters were coated is one of those trade secret techniques, details of which are difficult to find in patents. Probably anyone alive who actually worked on coating drums is in the upper 80s to 90 + years old by now.

I am sure spraying is how the drum was originally coated. However, I think high-build primer and automotive clear coat will not be a good choice. If you have only 0.001" to play with, you want a coating that is designed to leave a very thin layer. Both the coatings you mentioned are exactly the opposite--intended to build up thick layers. I think you will find it almost impossible to get a variation in thickness of less than 0.001" with those coatings, no matter how much you polish them. Maybe a very thin nitrocellulose lacquer, applied with an airbrush? Has the advantage of being easily removable with lacquer thinner if you mess it up. Also, as long as the drum is already on a precision spindle, I would not take it off and move it to a lathe. A lathe will not, in any case, give you the surface finish you need. Almost certainly that is a ground and polished finish. I would think the best way to restore it would be to spin the drum on its bearings while applying successively finer grits of abrasive papers, films, and polishing compounds.

1:00 I finally made it to the System Source museum this past summer, and it’s fantastic. I recommend booking a tour, but plan extra time! I was also finally able to confirm that one machine on display used to belong to me! Bob took a bunch of old Macs (and accessories and magazines) off my hands when I was packing to move abroad from Maryland, and had to get rid of stuff. I couldn’t be more delighted that it all went to a museum rather than the dumpster.

Machining the drum on a lathe with required precision will be really REALLY difficult, considering that there's no room for error. You should try a solvent or paint stripper first. Then just polish the surface (if needed)

Tim Hunkin has a remastered Version of "The Secret Life of the Videorecorder" on his channel where at the end he talks a bit about the famous stickytape and rust experiment and how they made the magnetic powder. Its not as simple as just using red iron oxide - if I remember right, you start with black iron oxide and heat it for a certain time to a certain temperature such that it partially converts into the red iron oxide and thats the stuff that can hold magnetization while being sensitive enough that the write head magnetic field is sufficient to make a recording.

I find it fascinating that harddrive tech went through a similar format progression as phonographs, from drums to platters

I noticed that the sides of the drum have the same magnetic coating as the main surface. This suggests that the drum was originally coated by being lowered in a trough filled with the magnetic material in liquid form (just the bottom of the drum) and then rotated to apply material to the entire surface. The centrifugal force would even out the coating over the entire surface of the drum. I suspect this was done in a sealed chamber to contain splattered liquid with some kind of mechanism to raise and lower the trough to start and stop the coating process.

I bet your plan for recoating the drum is exactly how Bendix did it back in the day. Low tech, hand made.

21:19 In my humble opinion, I believe that an effective method to repair the drum would be to deposit a ferromagnetic material on the surface through a vapor system inside a closed container, something similar to the method used to create optical mirrors for telescopes.

I love these videos, because it has 2 things I love: Vintage Computers, and learning about Vintage Computers. Keep up the good work, man!

Regarding the re-coating of the drum, I was reminded of an old episode of the Secret Life of Machines. Luckily, _Tim Hunkin_ (KZfaq) remastered these old episodes from the 90's and the Video recorder episode is available to review. Tim attempted to re-create a magnetic film, with some success, and he also talked about how the formulas for the media were a "special sauce" with each company having their own secret formula. I would also highly recommend Tims other videos, highly entertaining. The original series are all great episodes. re: re-coating method - my thoughts would be to try some type of electrostatic coating, similar to powder coating... Cheers,

You should consider an electrostatic process to apply the magnetic material to the aluminum drum. In the first stage a mixture of magnetic powder and some kind of PTFE/acrylic dust is sprayed on the drum surface while it is rotating. In the second stage the mixture is "baked" at certain temperature to a solid ring which sticks to the drum. Then you need an optical polishing tool with micrometer adjustment mounted at the read/write head position to get the drum running true before mounting the heads. No easy task.😁

There's Drum Head Alignment screws. Maybe all of these heads should be pre-set to highest elevated state above the magnetic surface before trying anything with the replacement drum and then probed individually to have them set to a good position. Just as a precaution.

I hate cleaning those resistors. I had them in the Navy on several systems as well as encountering them later on older manufacturing machines. Also, I find it interesting that in more advanced times you find that the clearances and specs of older hardware are actually quite tolerant to the extent that .0001 mil isn't necessarily required. I'm looking forward to seeing your success with recoating the drum as I think (empirically, not based on scientific evidence) that the result might be a bit easier than expected. Either way, thanks for an absolutely great series looking at these older technologies. Some of these I've seen but wasn't given a chance to touch them.

Have you considered PVD (physical vapor deposition) of a ferrous oxide coating? It would give you a far thinner and more even coating. You may be able to find a local company that could handle that for you.

Check the bearings on the drum that might also be some of the cause of the head crashes too.

The Applied Science guy probably can figure it out. He has experience in machining and chemistry.

You can really see why discs "beat" drums in the long run... so much easier to spin-coat a disc than a drum! Ooooh LOOK! BUNNY!

The method of spraying on a suitably ferromagnetic material should work to be fair. Just turn the drum between centers on a lathe. (the axel on the drum has visible signs that the axel at least were made in that fashion.) And beyond that it is as you say a simple spray coating application at a steady RPM. And remember to keep the lathe running while it dries, don't even stop it or change its speed. Since any changes will give the paint time to settle unevenly. I have a strong feeling that it will work fairly flawlessly. Might though have to play around with it a few times to get to the correct coating thickness. Ie don't be afraid to put a micrometer to the surface for the first couple of runs, and document both RPM, spray time, air pressure, distance and the viscosity of your DIY magnetic paint. Since changing any of those will change the thickness. Technically to get an even more even coating you would also have to oscillate the spray nozzle back and forth along the axis of rotation. Since te application will have a somewhat bell curve-ish distribution. (However, this effect might be too small to matter, but might be worth keeping in mind.) Though, another approach is to just coat it thick and cut/"grind"/polish it down to the desired thickness, but I have no clue how that would fair in practice, paint usually isn't the "best" material to machine. (Though, a customer at work grinds rubber parts to +/- 5 µm, and rubber isn't "nice to grind".)

I see you can buy replacement Bendix drums in the UK. Appears to be for a washing machine though 😂

I live only an hour away from System Source. I didn't even know it existed. I will be planning to visit now.

That's such a cool old computer, I grew up working on old tube amps with my dad so anything full of vacuum tubes has a special place in my heart. That thing is a beast, I'd love to see it in person so i could feel the heat coming off all those tubes while its in operation.

I agree with your decision to avoid tape. Wind aside, that drum's surface will see acceleration in the ballpark of a few hundred g, directly outwards. And the layer needs to withstand both its own weight (at a few hundred g), and the stretching of its entire layer - and not only must it stay intact, but it can only stretch out so much before crashing. It's probably not very much total force, but it's enough that if "correct" materials and techniques are an option, they are mandatory. And automotive coatings sound (at least to me), much more likely to work than tape. That said, I'd still look around to see if there's anything else within your budget that's actually rated for this kind of load. And definitely would be careful about actually machining the coating. If the bond is weaker than the coating, it'll be pretty hard to machine without flaking off.

This Bendix computer keeps getting more interesting. Since I have to start thinking about simple but modern digital servo processors for a project; AND I tried to build a EM relay/diode computer long ago (didn't have enough relays), AND the Bendix magnetic drum/ punch tape/cards/architecture are too interesting together for too many reasons... you've got me hooked! I built a weirdly efficient accumulator/shifter/base 10 to binary converter in one memory register and began designing the rest of a processor and realized I'd need way over 500 to make it work so I put it on hold 20 years ago.

The sockets exposed around 4:55 when you pull those relays ARE actually vacuum tube sockets - these were one of four styles used before the introduction of octal / locktal base tubes (the others had 4, 6, or 7 pins, but had two larger pins for keying purposes, and were usually assigned to the filament - the 6E5 magic eye tube is probably the most common example of these early tube bases).

Couple ideas for ya for the refinishing of that drum. 1) Check out Fiber optic Polishing paper for your final polish before coating. 2) Would Electrostatic coating be an option? 3) Spray while spinning in a negitive pressure atmosphere? 4) coat heavy and sand/finish to precision? - interesting to see how this all goes, thanks for sharing this adventure!

Man, it's so mind-blowing to see all those circuit components wired together from a time before circuit boards existed (or at least had made it into wide use)!

The squeegee is called a doctor blade. Has been used in the printing industry for many decades. There are factory notes and patent information with how to spray that drum somewhere. The coating formula/method would hold a patent.

I was going to actually recommend a good spray coat of ferrous paint mix and polish it smooth. So stoked that you are entertaining this idea, have often wondered if it would work to recoat a drum or platter.

can't wait to see the re-coating process!

Take some old vhs tape, wash away the base with acetone/xylene and use the remainder as your magnetic dope for the 2-part paint. Spin the drum as you spray, then polish, repeat. then use a lathe to dimension if better tolerances are needed.

4:03 You need some Texwipes and isopropyl alcohol or freon. No residue but it will remove ink and some paint. 12:44 Be careful, products with ammonia can have adverse affects on rotating memory devices. It can delaminate the coating on the drum or platters. Head crashes after housekeeping mopped. 15:08 The scent is acrid with a little ozone. 22:15 That drum will become gyroscopic if it spins up with any speed. A couple different companies make magnetic paint. A ride on the way back machine was fun. Nice video, good fun.

I have no idea if your plan to recoat the drum will work, but I’m delighted that someone is taking the time to think about this stuff. Such restoration has the potential to keep these machines running for decades to come, if not centuries.

I don't think the 2K clear will level the way you think it will, I would plan on applying it thick and turning it down the way you plan on doing the primer. Then there's the issue of 'are the crashed heads salvageable?'

Just remember to mill the rust into an ultra fine powder, best done in the binding material you'll be using.

I'm going to be the oddball here and suggest making a solid state drum emulator and get the rest of the system up and running before tackling a drum rebuild.

I was one of the commenters who suggested wrapping the drum in a spiral of audio tape. Now that I know the crazy runout specs you need to meet, I think you are correct in assessing the likelihood of success of that method. However, I think the spray method is even less likely to succeed unless you are willing to devote an insane amount of time just to developing the proper techniques, fixtures and materials. Among the things to consider: 1. Magnetic recording coatings aren't just "rust." They are carefully engineered blends of oxides with different chemical formulations (don't forget, Iron can be either +2 or +3), different crystal structures, annealing processes, etc. Do you know what kind of coercivity you need? What other magnetic properties? 2. the particles need to be milled to an exceptionally fine, and consistent size. 3. How can you get enough density of solids in your paint, and still have a coating that levels to a flatness of 0.0001" or so? 4. How do you control the application to get a coating thickness consistent to 0.0001" or so? 5. How to get a coating that will last another 70 years, without flaking off and destroying the heads? 6. All the things I haven't thought of yet. Still, pulling it off would be a real tour-de-force!

Debugging this system if one of the cards won't work would be quite a task, I'm glad that scope breakout is there.

My best suggestion for recoating that drum head, is spin it underneath a surface grinder to get it perfectly rounded again, and then electroplate iron onto it for an ultra thin ultra precise ferromagnetic layer

Very enjoyable series. I appreciate the enthusiasm.

Dann u guys comment quick

Glad to see more on the Bendix!

I have just watched a article on the Computer History channel called, Origin of the Univac 1103A, at 13:01 there is a picture of a drum being coated. Looks like it was done with the drum mounted in a lathe, with an air brush in the tool post.

I'm in agreement with others here: consider getting it precision machined and/or built up to exact tolerances as a pristine cylinder, first. Heck, you might find another youtuber has the tools for the job (lots of machinists on this site). As for the coating, the sides look like they're covered in the same stuff. I think that's a clue as to how this was done in the first place and is worth considering. Also, you can take a sample from that side area and test it with solvents to get an idea of what was used in the first place. Lastly, it might have been spin coated anyway despite being a cylinder (and making a colossal mess in the process).

Thank you for the fantastic video. Well done!

Did you figure out what went wrong with the old ‘scope? This is a cool series. Thanks for taking us along!

19:00 - you're basically talking about Bernoulli Principal, with the B Effect ripping the tape off the drum. It's an early JAZZ drive, without the cartridge.

The lathe idea sounds good, if it doesn’t work you could try mixing a magnetic compound in the form of a paste, apply it thicker than needed and then use the lathe with a scraper set to the exact right running distance, if then run at a slow speed in the lathe, the excess material could be gently scraped off leaving a perfect magnetic surface

Looking at the thumbnail, I now understand the ‘mouse-o-matic’ in-house nickname given to a make of drum memory that a mouse once got caught in (and let’s be honest - died).

If the red neck method doesn't work out (im all for seeing that!), maybe give electrolysis a thought? Since the flywheel is a conducting metal, it might be possible.

OK, Usagi, you have totally got me down in this rabbit hole--pun certainly intended. It's clear that microscopic examination of the coating is valuable. Don't strip the old coating off the drum until you have developed your process and are ready to coat--you may want it for microscopic comparisons. Particle sizes are sub-micron, so you will be pushing the limits of a good light microscope to resolve individual particles, but clearly there is still a lot you can learn about how they are dispersed and bound. One resource (Kreiselmaier) suggests making tests on glass microscope slides, which had also occurred to me as a good inexpensive source of test blanks.

how about ~20 000 grit paper/stone to bring leveled surface. how about CrO2 Polymer Resins: Various polymer resins are commonly used as the base for the binder. These resins provide adhesion and binding properties to hold the magnetic particles together and adhere them to the tape's base film. Solvents: Solvents are used to dissolve the polymer resins and make them into a liquid form suitable for coating. Common solvents include toluene, xylene, and various alcohols. Plasticizers: Plasticizers are added to improve the flexibility and durability of the magnetic coating. They help ensure that the coating remains pliable and resistant to cracking over time. Surfactants: Surfactants can be added to improve the spreadability and wetting properties of the coating mixture. They help the coating material spread evenly across the base film. Antistatic Agents: To minimize the buildup of static electricity on the tape's surface, antistatic agents are often added.

Electroplating is a common metal finishing technique that can achieve each of these objectives. This process involves immersing the aluminum workpiece into an electrolytic solution and introducing an electric current to deposit dissolved ions of another metal onto the surface.

Your enthusiasm is infectious 👌🏻👍. Follows with great interest!

you should experiment re coating the hawk drive platters , and get them to read before trying the drum

Thanks you for another fantastic video! We saw the sister unit at the Computer Museum @ System Source and Bob told us about what you're doing. I can't wait to see it running when you finish it-he put the best man on the job for it!

Thanks for talking about the bendix ❤

Pretty cool video, thanks!

Nice. This brings retrocomputing to a whole new level... correction: to an even _older_ level! 😊

Good luck with the re-coating effort. Really wish it is a success❤❤❤

As a mechanical engineer, your solution for manufacturing the coating sounds very good. I would give it a shot!

There may be small chance that there may be enough thickness of material just to true the drum. Refurbishment might have been part of their design. Though, I suspect that after the drum was sprayed, they polished it using a lathe. Though, finger sanding marks, such as those seen in bad bodywork would also be bad for the drum. It's possible that after the drum was coated that they merely, passed a loop of 1,000 to 2,000 grit paper under the wheel and gently lifted both ends up. Machinists sometime use this technique to get that last thou when their machines won't otherwise let them. If you want it shiny, there are telescope grits such as 5-micron aluminum-oxide and 1-micron cerium oxide available. I suppose that it could be charged into a stiff block of felt, held in a lathe toolpost. As far as spraying it, the challenge is: the proportion of binders to media. It would seem that there needs to be a lot of magnetic material in proportion to the binders. Making sure the spray mixture is well strained is a thing. Party trivia: The circular nut is a DIN 1804. Amateurs without the proper tool mar with a steel chisel to remove it; but a pro without the proper tool will use a piece soft metal such aluminum or brass and not mar it--or you can just make or buy the tool. You could make one.

Look forward to these every week! I’m thinking you would need an incredibly accurate lathe when you’re talking those tolerances? Can’t wait to continue the journey 🏆

It's strange hearing an American use the word "muntered"...... I love this channel

with a name like "Bendix", it reminds me a bit of futurama. Usagi: "I'll get this thing to work" Bendix: "Bite my shiny metal #@^" (The bot "Bender" on Futurama, that is)

Tape is definitely a non-starter. Spraying a ferrous coating is the way to go. Good luck!

I was also thinking of spraypainting the material on before you made mention of spraypainting. This should be a good way to get it fixed.

Very nice construction, almost avionics quality workmanship

When you come back to Hunt Valley, if you are into BBQ, try out Andy Nelson's BBQ! They are right down the street from Bob's museum, and it is possibly the best restaurant in all of Maryland! Andy shows up sometimes. If he does, ask him about his time as a Baltimore Colts player! Seriously... if you don't go, you'll miss out. 🤘

tim of the secret life of machines had some insight into making their own magnetic tape on the video about the VCR during the little after show segment he did on it. They made their own plastic audio tape by using some tape and sprinkling iron oxide on it. The interesting thing he mentions is that raw iron oxide did not work, and it had to be processed before it would actually work. It's been a year or so since I saw it but I remember him saying something about having to cook/heat it first. Might be worth a watch

When you said about the new coating, my first and only thought was about PVD. I'm sure there are many enthusiasts here who have their own vacuum chamber and can sputter thin films. I don't know much about these particular coatings, but I think it is possible to make em using PVD.

Magnetic paint is a commercial product (FerroPaint was the first that came up) - i'd probably take a chance on some of that rather than punting the binder will be compatible

Oh, and it's so good to see such a boopable snoot at the end your video! That's a cute little bunny-rabbit!

One thing to consider is the size of the magnetic particles that you use. Ideally they should be in the range of 10 micron or smaller. The smaller the particles, the better performance of the media.

card reader machine seems like an image scanner, or course you electroplate or vacuum deposit a layer, then lathe it, or lathe it and then coat, of course you use the same method of manufacture as the drum is being written/read, even electro-discharge-machining in liquid.

Kudos to you for digging into the messed up drum and considering how you could repair it. I had a feeling that would be an issue when you first mentioned how everything works off a magnetic drum that had timing signals written on it. That should be fun to rewrite on the drum after a recoat👍. lucky that you have another one to test with. At worst, i'm sure you could emulate it. Very much looking forward to this and I would love to see the recoating attempt.

Despite being less involved than a transitor, I somehow fail to picture how tubes even manage to do any computations. Might be because I was born in the late 90s and these machines don't even look like a computer to me. Not as exciting as the Centureon in terms of seeing an old computer running, but definitely just as exciting seeing a historically significant computer getting to work again. No offense to vacuum tube or mechanical computers, but they just don't interest me in the same way. They're less a computer and more a giant machine lol. It's more akin to somebody repairing old machinery, I'm not even gonna bother picturing HOW it works (like I can for the Centureon and others), but see that it actually works at all, otherwise I'll go crazy lol. Hopefully this project has smooth sailing. My hats off to you for even getting into computing this far removed from today.

I think spraying will work, but you should keep the drum spinning at a constant low rpm while it dries (like over night, etc.) to prevent drips or the top from being thicker than the bottom, etc. Also you may want to do it in a mostly clean room environment to keep dust from being hardened into the clear coat.

I reckon the spray paint plan is doable, can't wait to see it

Fingers crossed the coating will be successful. I want to see this beast up and running in all its beauty :D

The drum has some important read only tracks though (for timing and such), so those would need to be written correctly on the recoated drum.

I think a collab between Applied Science and/or Ox Tools could be productive.

I think that surface will have to be repaired 'in situ' doe to centering tolerances, assuming the bearings are perfect and lubricated. Use the bolt holes of that cover to position a tool to smooth the surface, and then a light spray in several goes of iron oxide primer should so the job. Always interesting how much mechanical precision was around back in the 1950s.

You can clean the drum with alum media at around 60-70psi should clear off old coating . When i have to do mission-critical parts and ceramic coat them, alum oxide is my goto

Your method for spraying the drum could definitely work, altough keeping your tolerance while cutting down the diameter back to bare metal will be difficult. keeping a 1 thousands of clearance is probably impossible with a normal lathe, maybe ask some machinist youtubers about the issue, they probably know way more than me, and could point you in the right direction. A fun challenge to say the least!

I would fabricate a way with a leadscrew that attaches to the drum unit as you currently have it assembled. Then use a very sharp steel cutter mounted on this contraption to turn off all the old oxide coating and aluminum oxide from water damage. Then take the way off, sand the surface of the drum for better adhesion, and spray the new oxide on as you described while spinning the drum and let it completely dry. Then reattach the way and turn the new oxide completely flat. After that disassemble the unit, take the drum out, and clean everything thoroughly to remove all dust and grinding residue from the housing and drum. This last step done with multiple cleaning baths to make sure all traces of dust are gone, while not getting any water into the bearings. Then lubricate the bearings and reassemble.

Would be fantastic to get consultancy from one companies that made their living from that, the one that make mechanic drives, or companies like basf that sure could work out the right process and chemestry. But as those are so gigantic, you can expect much. Anyway, this is a work of art, not a computer. It is deligthfull

Recoating drums was a farily common thing in the early years. I remember reading accounts in various computer history articles and books on early machines. I would search for articles by such people as Conway Berners-Lee (the father of Tim Berners-lee, inventor of the WWW) and Tony Hoare. "Washing the data off" was a thing, especially when a machine was used for "sensitive" (e.g. military"). They would wash the oxide layer off with something like acetone and then recoat, which may well have involved dipping and rotating the drum in new coating in liquid form. Machining off the old oxide is almost certainly going to end in nightmares, or at least tears - much to aggressive (but arguably very Texan...).

Too bad I live several thousand miles away, I would like to go along. Fascinating project 👍

Here’s my thoughts: 1) I wonder if you can turn the coated drum? If so, you don’t have to worry too much about making the coat even, as long as it’s thick enough; just coat messily and then turn it down to final dimensions. 2) I say “turn” but I suspect you’ll need better accuracy than is typical for a lathe. I think maybe you need a surface grinder that is setup for polishing cylinders (something I have heard of but never seen, speaking as an armchair/KZfaq machinist) 3)I worry about the dynamic balance of the drum. I think you’ll need to do something to balance it after all the turning and coating. 4) you need a test rig and prototype to test processes. Something like a 2-inch puck of Al, a motor to run it, and a read/write head. Bonus points if we can test the real drum in the test rig somehow to benchmark how well the rig simulates the original.

I think your coating idea is going to work. That's exactly what i was going to suggest. I would consider doing all your machining on the original bearings because then you don't have to worry about centering error, etc. Take the heads off, clamp the whole thing down to your lathe bed, and spin it with an electric motor.