The Tenifer treatment Glock does is supposed to go down into the pores to protect the metal. You must live near the coast to be getting rust.
Sending it back to Glock for refinish would probably be the least costly method.
For not much more you could have someone local Cerakote the slide.
I had a G19 that someone had polished to make the slide look like nickel.
I Cerakoted it silver, so it wasn't so shiny and had some corrosion protection.
My current Glock is a G48 (slimline 9mm). It came with the silver colored slide which I didn't mind at all, but I did it in FDE. What can I say, I just like FDE.
I did my .45 Shield in a darker FDE, "Smith and Wesson Tan", same color as the factory uses.