Update: I quit using filler on door edges after 12 years and I was wrong to wait so long
I always figured skim coating door edges with filler was the only way to get them straight, but a guy named Ray at the shop in Springfield showed me how to work the metal with a hammer and dolly and a shrinking disc, and after 3 panels I'm getting edges flatter than anything I ever got with filler, so what are you all using on door edges these days?