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Should brass oxide be chemically removed prior to painting brass with an enamel paint?

I don’t think the oxide is an issue for paint. What will be an issue is any residual oils. The usual solvent degreaser doesn’t do the whole job—not even acetone. After the solvent, it needs to be cleaned such that the surface will fully wet. Ammonia works well—even Windex—or rubbing alcohol. Apply a self-etching primer as soon as it fully dries. Then paint with enamel paint. The etching primer should eat though any oxide—that’s why you do it.

If you are talking about true fired vitreous enamel, then I would suggest it will be difficult. Watch dials have been enameled and are often made of brass, but it’s considered an art and requires very careful temperature control during firing. Even the experts have a high rejection rate. It’s easier on copper and silver. Oxides would be death to that process just as with soldering and brazing. But worse is not following the correct cooling protocol, which will cause the brittle enamel to crack. When done right, though, it’s gorgeous. Cloisonné is a form of this process.

Rick “who would probably remove the oxide just because” Denney

I'm just talking about these rattle can enamels which have become remarkably good and remarkably resilient over the last couple of decades. Even their little spray nozzles have become darn good. From what you're telling me, I'm thinking if I just gave a raw brass instrument and acid bath, it would remove surface oils and oxidation as well as etching it a bit for good bonding. Do you disagree with this?

No I don’t disagree—etching primers also use an acid for that purpose.

Rick “wetting ability is most important for adhesion” Denney