I did a similar job on my black guitar, I dropped it when incredibly drunk and bodged a big chip out of the enamelled finish. When I took it to be repaired they said it would be frightening amount to fix it as they would have to do the whole bit. The guy then smiled and recommended I go to an art store and buy some high gloss black enamel, touch up the gap with plaster and paint over. I did, and it did the job!