Hey Bucky. I have been in your predicament before. Did a lot of studying. Now, from what I have learned, regular body filler
will NOT work. Powder coating requires the parts to be heated to 450 degrees, if my memory serves. Body filler cannot withstand
that type of heat. I think it shrinks. I had a tank powdercoated one time, and it turned out awesome. However, it was straight as an
arrow. I think that is really the only way to go. Try scouring Ebay for a tank. Try to find one advertised as no dents, and make sure it
has the original paint on it, so your chances of finding bondo underneath are greatly decreased. That's your best bet. Now, one time I
also had a frame powder coated, that had some bad imperfections in it. I filled them JB Weld, and smoothed them out. The powder coating
guy said he would make no guarantees that it would hold up, but it did. It worked great. So for small imperfections, I would do that again
with confidence. As a last ditch option, upon searching the internet for fillers that would hold up to the heat, I did find one that claimed it
could do so. Don't remember the name, sorry. But if you do a google search for powdercoat body filler, or something, you might find one
that will work. Hope that helps!