Well, here are a couple of snap-shots of my non-Archer Etchant "rust." It perhaps isn't as realistic as using etchant, but it's safer... maybe
I paint the roofing with Floquil Foundation and Earth. Then I paint a "slurry" of alcohol and ground chalks over that.

I paint the roofing with Floquil Foundation and Earth. Then I paint a "slurry" of alcohol and ground chalks over that.