copper or brass as a backer for weld filling?

[Mark Andy <mark@sccaprepared.com>]

Howdy,

I was talking to a friend yesterday who's a fabricator, and mentioned that 
I was going to be starting to do some sheet metal patching work in the 
racecar to fill up some of the various holes in the firewall, etc.

He mentioned that I should use a chunk of (copper or brass, I can't 
remember) as a backer on holes up to around 3/8" and then I could just 
fill them up against that, the weld metal wouldn't stick to the (copper or 
brass, I can't remember), and it'd leave one side nice and smooth.

I got some copper and some brass...  Which do I use?

Thanks!

Mark