welding... rollpan

A friend of mine welded his roll pan up over at my house... I help him bondo and smooth everything.. This thing was on nice and strong I stood on it and nothing flexed.. We got it all bondo'd and smoothed primed it and on his way he went... a week later the bondo has cracks in it... Would it be because of his welding?? Im just curious what went wrong.. Any ideas??

did u prep the metal right..

We grinded down all the paint and primer and only welded raw metal to raw metal.. but for somereason it still cracked

Is the bed lacking structure anywhere else? Like is the tailgate missing, or is he hopping/ railing his truck, or is is slapping the bumpstops when he drives?
Other than that, if the filler was spread too thick, it could have shrunk as it cured and cracked... lots of reasons it happened- good luck figuring it out so it doesn't happen again!

the usual cause of cracked rollpans and tailgates is because when people weld them up..then make it fit to tight and weld over that seam and then when u grind it off..u got no metal left to hold it together and after u drive it it cracks the weld joint...u need to leave a gap about the size of a cutoff wheel where your welding your pieces together so the weld can flow into it..that way after its ground down u still have something left for strength

Well damn... He is rolling without a tail gate he does hit bump stops and it was butted up pretty tight when he welded... Thanks for all the help... and when your tack welding you need the metal close dont you?? Or should you start your puddle on one side and go across the gap??

welding part dostn really mater as longest is a good tack..but like phil and trevor sayd

Wouldnt kitty fur or something like this also help in the flexing of the metal?

kitty hair is what you so post to use ..period good shit lil hard to sand but worth it

Doesn't hurt to weld down the backside of it either.