Body welding question.

I did some searching here concerning welding up holes in the body. Several post mention using a peice of brass or some other kind of metal behind the weld. What would this do, I guess help hold the weld up until it set's ? My 66 has lots of holes under the doors as expected for a deer lease jeep and I have lots of welding to do. I do weld but am not an expert so any help would be great. I love this site, need to post pics of my rust bucket. Thanks again, Doug

 

Prevents too much heat from building up and warping the sheet metal.

 

The idea is that the lump of brass (I use a square head brass hammer) acts as a heat sink, so the heat stays concentrated right where you want it (as MOP says), and also, brass won't stick to your weld.

 

well I just spent 4 months welding up holes on my 49' never welded before so any hole was almost impossible for me to weld without a 1/4" think 2"x4" flat stock piece of copper..worked great since the metal is so thin keeps you from burning through even with the welder on the lowest possible setting.. this technique worked for holes up to about 3/8" or 1/2" if you want to use a LOT of feed. the larger holes however still warped due to the heat even when using the copper.

 

Mojave, I haven't bought a brass hammer in some time, are the expensive and is there an alternative to buying a hammer or would somelse work. And thanks to all the rest of you who responded.