The ash tray is full, time to get a new car.
You know, why worry about cleaning
the bottom when you can buy a clean new boat?
The piston coat sounds pretty much like generic Teflon material. It all (all) is "goop" that gets baked to set it, same same as it was applied to your frying pan or anything else. The trick is, not many vendors will just sell you a single
pint (or gallon) they'd rather offer you their "application services" and charge for the entire job.
On a prop, I'm told that normal cavitation will pick it off the ends of blades and then start it decaying back from there. But if that piston stuff isn't priced like gold...it might be worth trying anyway.