I'm new here, but just noticed your post because I am doing a similar thing. The powdercoater that I've used before told me that lead solder will melt at the temperature used for powdercoating, that whatever you use to fill has to have a melting temperature at least 60% above the temperature used for powdercoating which is usually around 400 degrees F, plus 60% gives you 640 degrees F. Lead solder melts around 400-500. He suggested silver solder, which depending on what you have melts at like 800-1100 or something. I think you can melt that with a MAPP torch.
JB weld sure sounds like a nice cheap alternative though!