I paint all of mine with basically engine machinists aluminum-cast paint and it holds up very well and doesnt flake off like regular paints do. I really dont see many transmissions that have been painted having trouble with paint coming off though but if you plan to stab a pressure washer wand right at it then its likely doing to start flaking off. If I were you I would not even bother powder coating the case, it is a LOT of prep and expense. A few years back a street rod trans I built- the guy wanted it burnished and powder coated. In the end it looked like we sprayed clear coat on the trans and wasnt worth the hassle. EVERY hole and internal access MUST be blocked off as the case needs blasted before powdercoated and takes a lot of prep time. It is very tough stuff but you arent going to see it much and the time and expense isnt worth the hassel IMHO. Now if it was a limited use show car I would give you a different answer but would probably still paint it as thats what most of the big shops do and it seems to work fine.
