It's been discussed on here many times -- how to smooth welds (ala frame welds) and still powdercoat but not via welding itself. (you can't powdercoat bondo!) I've seen various metal content fillers before used with moderate success, but found one today that comes highly recommended via a powdercoating facility that does top notch work in the Seattle area.
They use a product called "Lab Metal". Two variations are "Lab Metal" itself which is good for 350/400 degrees (just have your powdercoater back off the oven a bit) and "Hi-Temp Lab Metal" which is good for 1000deg. link -->
Alvin Products Various methods to apply and then file/sand as required to polish out.
One cavaet though -- this isn't a replacement for actual welding -- it's purely for cosmetic purposes only. Also talk with your local powdercoater BEFORE you do this -- make sure they are comfortable with what you are doing first. Not all powdercoaters are the same!
