i orginally had a plate welded in there but when i was grinding the weld down it got to thin and had to be cut out and my question is if i fill that with bondo will it crack out? it would only be about 1/16 thick and i plan to lay some fiberglass on the back side of the fire wall to usure a good seal from water and what not.

on the left is what it used to be and on the right is what it is now with the plate taped fast on the back side of the firewall

 

i think as long as you have a couple layers of fiberglass backing up the bondo it wont crack

 

ok this is what i was thinking too but i just wanted to double check. thanks by the way

 

why not cut it out and put a new piece of metal in?

 

This is something I'm gonna be getting into soon as well. I've made up my mind that I'm going to weld some sheet to the inside, and either fill the whole thing with bondo, or use seam sealer around the edges and see if I can make it look like a dimple in the firewall.
I can't see why either of the two wouldn't work.

 

u need to put kittyhair over the place u weld....this way u fill all the small pinholes from welding with something that is water proof.......just reweld it from the inside so u dont have to grind it as much.......and remember to just spot weld all the way around the panel u are welding to make sure u dont warp the metal a whole lot......as a rule of thumb u should always use kittyhair first,sand that down flush then bondo and flow filler