On Sat, 23 Apr 2011 12:00:48 -0700 (PDT), whit3rd
wrote:

On Friday, April 22, 2011 7:51:24 PM UTC-7, Califbill wrote:
I am doing some rebuild on my aluminum river jet boat. Going to weld closed
some old bolt holes in the gunnel and dash. What is a good backing plate

Copper contamination would be bad (it accelerates aluminum corrosion),
so I'd consider... ceramic. A brick, a coffee cup... something like that.
You don't melt the copper. All the aluminum welders I know use copper
backers when welding gaps in aluminum.