That’s correct - that’s why I diluted the spray bottle in the video with water. That’s what I like to do for mower decks

Yeah none of these Super Clean products are what you’d want for under the deck. The underside of the deck is best dealt with by using a metal putty knife and a pressure washer. That old dried grass ends up like clay caked on under the mower. That’s the best way I’ve found to deal with it. And to keep your mower from rotting out and collecting grass like that, you should rinse your deck off with your hose or using the cleaning attachment on your deck (newer mowers) every 1-2 mows.