Use Terpentine. It will take you 2 min. max. Very effective. Use small amount, barely to wet the soft cloth. I use it with my bike all the time. It dissolves everything from residual glue from stickers on the frame to degreasing components I need to clean and lubricate again. I have never had anything that it can't deal with so far. I don't believe it's gonna damage the copper base of the cooler.