I agree with Albert, Sharpie is the best, and often works wonders. I've picked up some pretty rough equipment used and have tried other options too. For gouges in paint that hit bare metal and can't be covered with a sharpie, you can touch them up with Testor model paint available at a hobby shop of craft store. The paint comes in 1 oz bottles - flat, semigloss, or gloss. You also need a fine tip model brush. Recognize that this will cover a scratch that's visible from a distance, but is likely to be noticable close up, and depending on your workmanship, it may look worse than the scratch.
