I had the same problem with Mamiya TLR. The paint on the edges of the back panel always coming off. I paint it after every use again with oil based black Rust Oleum paint for metals and it coming off after next outing. Last time I Paint it with Rust Oleum white oil based primer first and after it dry with the same black paint. It was a year ago and so far it stays. Another good option I think could be the touch up black paint for appliances, like refrigerators, washing machines. It available in the Home Depot in a little plastic cans same size like a nail polish and it is an epoxy based, so it may have a very strong adhesion. But i didn't try it yet.