Fuck. I've made this mistake before and I don't want to make it again. When mounting paper onto museum board or wood, there IS NO REPLACEMENT for matte medium.
I was almost out of matte medium so I used a mix of matte medium and clear gesso instead. F-. Two days of transferring down the drain. The problem with clear gesso is that it's grainy texture just doesn't cement paper to other substrates the way matte medium does, leaving large bubbles that appear when the front of the paper gets wet.
Learn from my mistake. Clear gesso is fine for the front, but only matte medium does the trick on the back.
Alright, time to re-transfer "The Bird" all over again. FML.