With a Triumph cutter I just purchased, I was given a spare blade that was apparently just sharpened before the machine was taken out of use a couple years ago. The actual leading edge looked ok, but the rest (including 3/8" onto the angled portion approaching the sharpened edge) was covered in rust. What causes this? (the blade in the machine is rust-free.)

Not knowing what to do, I removed the rust and sprayed it with oil that says "displaces moisture and inhibits rust" on the can hoping to keep it ready for use, but the rust returned in 2 days already!

What causes this? Can I fix this and keep the blade from rusting so quickly/badly?

There used to be a product called Rust Eater some years back, or you could try a marine shop or chandlers for rust inhibitor gel. Are you on the coast or did your machine come from a coastal location?

Thanks very much Richard - will seek out Rust Eater or buy a rust inhibitor gel online. I honestly don't know much about protecting steel such as in these blades. It proplexed me how much this one blade rusted while the other is fine (and blades in our small stack trimmers have not rusted one bit in the last couple years)

I'm on an island in Lake Michigan so there is some humidity, but usually things don't rust too badly here. When I got the cutter in July it came from Miami Florida and this spare blade marked "sharp" from a sharpener in Florida was already covered in rust so that climate must be a lot worse for things rusting.

So far the oil might have done some good as within 1 day it changed from the cleaned dark steel color to rust brown color which again perplexed me, but it hasn't yet developed any texture which is at least good - but I need to do more as I don't see wanting to put the blade into the machine that would potentially develop scale that could work its way into the machine and/or discolor the edge being cut.

Rust is a chemical reaction, a very slow one. You need to get rid of all the rust and the oxidized layers of steel that are about to "decompose." After that some sort of protective coating.

If it came from Miami that is really bad, anything in a humid place near salt-water will rust like crazy.

Talk to your blade sharpening people, they must know something.