I think that it was terrapin who said that Fusion is very good paint for plastic.
I have a can of brown that I have almost used on a Model M case several times. Once I dye a set of keys brown, I will do the case.
My testimonial to Dupli-Color Vinyl and Fabric Coating ($8 at NAPA) is pretty solid, however.
I did 2 cases in Charcoal Gray on my Model F 122-key terminal boards a year ago and they have held up flawlessly. They remain parked on the desk 98% of the time and I am careful with them (eg clean hands and no food or drink), but they are exhibiting no wear at all.