The "problem" with powder coating jewelry, with the possible exception of chunky bracelets, is the ungainly thickness of the coating.
A far better solution, if you can afford it, is to have your pieces e-coated. For lack if a more technical answer, think of it as a clear plating solution. Works great and is usually worth the money, if you consistently have tarnish issues.
If casting, look into the many tarnish-resistant casting alloys. You'll be impressed.
If fabricating, think about Argentium and some of the other company-specific anti-tarnish wire and sheet alloys. You'll pay a small premium over 925 sterling, but you'll save a whole lot in firescale issues and after-sale tarnish complaints.
David L. Feldman Consulting