Thank you for this suggestion.I would just touch it up the scratch with 2 coats of Testors Matte Black Model paint, letting it dry overnight between coats and then wrap the chain stay with a neoprene & velcro chainstay protector to cover the scratch. I put one of these on all my bikes anyway. Here is a link to a good one from the Netherlands
Buy Lizzardskins Chainstay Protector Neoprene Large - Black
Order the Lizzardskins Chainstay Protector Neoprene Large - Blackhollandbikeshop.com