Dude, that's what filler is for. The welds do not have to be "invisibly smooth". When I weld in a patch panel, no matter how good of a job I did, it still needs a little filler. After welding in a patch or a whole panel section, I always grind the welds a smooth and flat as possible, and then I apply a thin layer of an All Metal type filler. It is waterproof. Then I sand it smooth and continue other bodywork from there.
