Today I finished fixing the horn buttton. It was scratched, the paint was partly gone and the rest was discolored. It took me a while to remove the rests of the old paint with a mild paint stripper, a load of cotton swabs and a lot of care. I sanded the inside with small pieces of sand paper 500, 1000, 1500, 2000 and finally 2500 untill all the scratches and pits from the paint remover were gone. After polishing the inside and outside I went to find the right colors of paint in the form of touch up pencils. Ultra small paint brushes and more care did the rest.
I am quite happy with the result.