I recently bought a cheap donor bike because it had the late '80's Deore components I needed. Everything was in good condition except for the rear derailleur; mechanically it was fine, but the painted finish on the outer plate was bubbled, cracked, and completely ugly. Using a cheap fingernail buffing kit from WalMart, I was able to make this old part look passable enough that I don't hate the idea of using it.

Nice! That is definitely an improvement.

Looks nice. Just buff it with some aluminum polish from time to time to keep it from tarnishing. I did the same with a Deore crankset of the same era. I wish I had a "before picture" but I didn't take any until after I media blasted them. Here is one with one crank arm media blasted and the other polished to a satin finish. I didn't take them to a high shine because it's just too hard to maintain and it was for a beater bike anyway.

Spray a coat of acrylic clear to prevent corrosion and maintain the finish (or polyurethane if you want a tougher finish).

To the OP, you can use clear nail polish for the the coating. Spraying will only gum up the moving parts.

If that's for real then serious kudos. Would it have worked for a black derailleur?

I don't see why not.