Dilute HCl should not affect a cyanotype. Some folks who have an alkaline
water add a little dilute HCl to the wash water to neutralize the base.
A base will bleach or fade a cyanotype. My question is, "Why do you
want to put a cyanotype/gum in HCl?
I have never put gum over cyanotype. I have put cyanotype over gum with
success. I'm not sure what the dichromate would do to the cyanotype image.
Gum can be put over platinium.
Bob Schramm