Polyvinyl alcohol - available from B and S. I put about 1 drops per 1ml of sensitizer/metal solution whenever I use platinotype, Uno or Lenox, which all seem to be more 'spongy' and absorbent than Platine and Whatman's, which are both heavily body-sized papers.

My working theory is that I need to keep the solution near the surface on the absorbent papers by using the PVA sizing and quick warm air drying. On the other hand, I allow it time to absorb into the heavily sized papers such as Platine and Whatman's by just letting them dry with ambient temperature and a little moving air.