I have used nail polish to fill in chips (don't tell anyone, but it worked rather well). I have also used a black base paint where the metal showed and then filled in with layers of shellac. It requires a bit of techniquie, but it's not difficult; a cotton rag you make into ball, fine dust or grit made for the purpose, and applying shellac several layers. You can use sand paper, the type that stands up to water, fill in the scratch or chip, then carefully fill in, one layer after another, and gently polish to even it out. There is a dedicated thread on how to restore the japanning on old sewing machines on the top of this forum, I guess that's the most thorough way about it. I have mostly cleaned up more or less grimy machines, then polished and improved with minor thouch ups like described.