Not sure how much. I wouldn't go 1/2 an inch, just maybe 1/4 will do fine. Also check out www.eastwoodco.com and they have a sealer there just for that. You get a can of it and brush it on and it's like rubber basically and it seals it all up. What I would do though when doing this is first spray the panels with weld through primer (so it dont rust again) then weld them together, etc.