Done a couple of these lately. one on my S2 turbo and spent ages drinding the welds back then leveling off with flexible filler before coating with seam sealer before painting. looks really good after painting

The other was on my S1 track car, that was piss easy as the car was already totally stripped, just buffed the welds a bit then dressed in seam sealer again before painting.

S2 turbo before seam sealer