a pro tip ...1. sand your master profile to desired finish before casting to the need profile.... 2 use a parting wax and or Vaseline instead of wax paper... 3. get some fine Aluminum Oxide of the desire grit. 4. mix Aluminum Oxide into you bondo 5 . add harder... 6 cast you sanding block forget about trying to contour the sand paper to the block.. trim your block and use you sanding block when hard

Genius

Thank you. I will try that.

Crisco-the white stuff-worked fine when I didn't have any Vaseline.

Consider fitting it back on the window really hard while glue is still tacky and form the sand paper into mold.