Seems to me that you have worked out a very satisfactory solution to a really big problem with powder horns. It looks good and I have seen untouched horns with similar 'spots.'
I have always thought that using dental bonding material would be a good fit for this, but never tried it. It is a two part resin the has to be cured with light. So, not being a dentist, I don't have these things and they may be expensive. The dental bonding material can be colored with additives and theoretically it should work well. It can be filed and shaped with ease, but it is pretty durable. Any dentists out there?
Dick