Make sure the steel is clean, no rust. Then use some rust bullet or Por-15 as a base. Then cover with paint as advised on the previous application.
Then I'd cover it with a tank pad once it is good and dry.
I used a dremel on a couple of spots on the roof of my truck (to ensure clean steel) then used Por-15 and then the matching auto paint in a bottle to dab it on.
As an example on the paint for the Concours -
http://www.colorrite.com/product/kawasaki-684-metallic-nocturne-blue-1609.cfmBut if the scratch doesn't go to the steel, then I would use a dremel or worry about the steel part.