You may want to try MAAS Metal Polishing Creme...sold practially everywhere, even 'Wallyworld'. it has worked better than std. Automotive Metal Polishing products. if you're concerned about scratches, I'd advise you to stay away from the Scotchbrite.
I was wondering if maybe steel wool would work. I've never welded before, but I've soldered with a torch. Steel wool and emery paper work well on other metals.
Chemicals or sprays wont remove the burn marks. They will have to be abraided mechanically. 3M Co. makes some products from nylon impregnated with a mineral that will clean those marks. The product is called "Scotchbrite" and is sold in rectangular pads or wheels that attach to an electric drill. These items should be found in an auto parts store or maybe a big box store.
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