I have a bunch of old cookie trays from home ec. or whatever it's called now. I have them put out their paint
and then at the end of the hour, spray the trays and cover with a small trash bag. They have to keep using them for the whole project--no washing. It makes some of them nuts but they accidentally get into some
interesting colors -- at the end we'll do a pallet knife painting and use up all the rest and then they'll all soak a tray and scrape it off.