If the paper has gelatin sizing, due to considerable duration of acid and water washes it is possible imo that the gelatin sizing might be diminished or even removed completely.
Afaik, many water color papers now used AKD sizing, and it is impossible to effect AKD sizing with acid treatment, but I have no experience with Zerkall papers. I have no idea on its sizing.
I tested Hahnemühle's gravure paper with argyrotype, its surface was very delicate and the print easily abraded, because it has no sizing.

I added ilfotol to sensitizers (argyrotype, new cyanotype, VDB) with SA treated papers, may be it can help with grainy highlights. I have no experience with Pd, so don't take my word for it.