Virtually all of my weapons have been metaloyed for just this reason. In any event, after a weapon has been cleaned and oiled, I seal them in a medical grade vacuum seal bag. In the 30 years I've been doing this, I've never had an issue with rust.
Yes, finishing can get expensive, but if you send in 3 or more, you can often get a "bundle" price.
I've got a gun, and I use that term lightly, a Llama 1911 I purchased back in the 80's, 3 gunsmiths couldn't get that $335 hunk a metal to work. Recently, a few months back, I had the entire weapon metaloyed and its been buried out back now since. Dig it up once a week to see how long its gonna last, expensive test yes, but I did try getting rid of it at one gunshow, for free, but no takers. I was gonna make a desk clock out of it, but looking at it daily would have been a reminder of its failure.