We just had a Ranchero chassis in the shop that the weld beads were not even on the seams in many places. I guess the welder was daydreaming or could not see where he was.We normally reweld a few places and clean up the factory welds , especially in the engine compartment section of the frame for appearance.
Frank, is correct about avoiding too much heat. Do small sections like with sheet metal repair or you could warp stuff.