Although my telescopes aren't beautifully finished like yours (that thing is pretty darn amazing!), I always have my parts powder coated, anodized, or use polyurethane.
Living in high and mostly dry Colorado I don't have nearly as much oxidation of unprotected metal parts. I have a friend who owns an 1892 6" Alvan Clark refractor (incredible optics) that is all unprotected brass and has a beautiful patina from many years of use. I've never been really happy with my own applications of spray on gloss polyurethanes, probably because I am impatient and clumsy as a painter, and so always use clear matt finishes. You on the other hand bus have great patience to turn out the finish quality that you have displayed in your 'scope. I am not very careful of the exterior finish of my scopes, which is why powdercoat is my choice. My optics on the other hand, are very well protected.
CHuck the grumpy old guy