I use a grinding wheel first on the welds, but it does take good control (and a very good face shield) to stay off the sheetmetal, then I move to a 5" round 36 grit disc and the same method....and finally down to a 60 grit 5" disc, then 80 grit on the DA. I start with Evercoat Metal to Metal for filler ( a very, very thin coat) then Evercoat fine strand glass impregnated filler, then regular filler, and last a glaze coat....

BE PATIENT!!!!!!!! A grinding disc, and the sanding discs generate a lot of heat and can warp and distort the metal very easily!!!! If you can't hold your hand on the metal, it's too hot!!!!