Welding Inside Car. The welding is to the outer sills the visible part. The video below.
Welding is most useful for bodywork and larger structural jobs. I would advise you invest in a good mig welder try a 2nd hand one as these are probably better than a cheap new one. Have checked the inner sills and even removed the carpet so the MOT tester can see inner sills.
Remove foams sealers adhesives and inserts from the welding area.
Patience is the name of the game. Maybe you discovered a cracked chassis a cracked muffler or a. Patience is the name of the game. Welding is most useful for bodywork and larger structural jobs.