I read on one of the other threads, that its best to do the test burn before you paint for a couple of reasons, #1 you won't have to repaint after you grind/weld if there is something that needs tweaked, #2 and maybe more importantly, they were saying if you painted your smoker while it was still warm, not hot, but still warm, that the paint would penetrate the pores in the metal and last longer.
I defer to anyone with more experience though, just sharing what I had read on a couple of other threads here.