Our plant has a powder coating department where we paint mostly aluminum. When we have buggers in the metal and a little body work is need, we use J-B Weld as a filler. It will take the 400°F for the 10 minutes it takes. Granted, we aren't painting truck frames, just small (2-foot x 5" x 3/4") parts, so if you have huge areas to fill I would imagine it could become a little pricey.
