recently painted the frame on my 521
rust-oleum rusty metal primer and rust-oleum semi gloss black with a spraygun. The truck is from so-cal, so my frame wasn't in bad shape, just surface scale. Wire cup on a grinder to clean most of the frame. I did sand blast in tighter spots, and finally brushed on some phosphoric acid (ospho, jasco, etc) on much of the frame before priming. I gave a quick wash of any areas treated with jasco prior to spraying, to make sure there wasn't any leftover un-consumed acid that'd show through after painting. Touchup later is simple and cheap. Wire wheel is cheap if your time isn't worth anything, but going over it all with a wire wheel is time consuming.