I did a lot of searching and reading last night. Some updated info:
Okay, beyond a doubt, confirmed it's the gun oil.
I need to revise this to "One of the causes is spraying the gun oil."
I've got a test piece on my workbench that I used the RemOil on again. It's just some old junk half-gauntlets that I never even put gloves in. However, this time I sprayed the oil on a rag, and then wiped it onto the piece; Sir Brian mentioned this on Facebook in regards to Sir Edward's gauntlets, and I realized I had sprayed the oil directly onto the armor ... just like with WD-40. However, I read that spraying *too much* oil onto the piece will cause it to *attract* moisture, and still rust. That *might* be part of the cause. I saw a number of people on MyArmoury who recommended RemOil at some point, including Mike Edelson. Nobody mentioned how they were applying it, though.
And yet, I found another odd event. I'm selling my SCA kit; I don't have any plans to go back to SCA, haven't for years, it's not period correct, but it was my first harness so I kept hanging onto it. I said I'd clean it up a bit before shipping. I cleaned the upper half of the harness last night, and went to do some research online. After a couple hours, I went back to the garage ... the armor had complete (but thin) surface rust! I didn't use any of the RemOil on it, just CLR and WD-40. I started sensing a pattern.
This sounds stupid, and I think it is, but ... I have a "blue" CLR bottle, and a "grey" CLR bottle. The blue CLR is what I cleaned my SCA stuff with last night - it's the older bottle, and I try to use the oldest stuff first. I'm not 100%, since it's only been about 12 hours, but, I'm about 90% certain the blue CLR is what makes the armor rust. It's stupid, I don't know what the difference is, other than the blue one is by "ZEP", and says "Professional strength". Neither bottle lists active ingredients, or even an ingredients list.
I cleaned one side of one of my pieces that was rusted a lot from the VARF demo with the grey CLR - and so far, no rust has come back at all. In contrast, the part with the blue CLR is already changing color. I've cleaned, once again, my SCA gear, with the grey CLR, and so far, so good. The test piece which was just wiped off and then RemOil'd with a cloth is still clean, too.
I also found some interesting comparisons between WD-40 / RemOil / Breakfree CLP and a few other brands. I'll post that later in a separate thread. Hopefully I've found what's rusting everything so rapidly; it was so obvious, since almost every cleaning I used the CLR and then tried WD-40, or RemOil, or wax, or just blasting it dry with the air compressor - and in every single one of those, I used CLR first. The exception was "Iron Out", which I mentioned above - using that in place of the CLR, it didn't rust, but it didn't have the cleaning power of the CLR on the heavier rust.
Sir Edward, what color is your bottle of CLR?
Edit: Here's the two blue and grey CLRs
http://www.homedepot.com/h_d1/N-5yc1v/R-100670289/h_d2/ProductDisplay?catalogId=10053&langId=-1&keyword=clr&storeId=10051
http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?productId=202751166&storeId=10051&langId=-1&catalogId=10053&MERCH=REC-_-SearchPLPHorizontal1-1-_-NA-_-202751166-_-N