Originally Posted by philldresser
Arches Aquarelle definitely appears to have an alkaline pH. An acid soak should improve its performance quite a bit with kallitype, or for that matter with Pt./Pd. I have not tried it with cyanotype, but all of the iron processes seem to like a slightly acidic paper.
Don't soak for too long, however, as the acid soak may break down the size, which result in a grainer, more textured look. But the size on Aquarelle is pretty tough, so maybe this won't happen.