Right, of course, I wasn't thinking about the PO in ionic form in solution.

That must mean that I don't have too much calcium in my water (even though it is city water from limestone spring/well sources, and is considered 'hard' by many people), as I don't see the problem in the prints at all.

However, if it is not soluble, how would it form crystals on the surface of the print? That would imply that it was in solution before the paper dried, correct?

It seems that the formation of the calcium oxalate might be in the first rinse or clearing bath after the PO, because any traces of free oxalates will disappear fairly quickly unless Mike is using oxalic acid as a clearing bath acid.

Any chance EDTA might bind up the calcium so it cannot react with the oxalates? I believe that is the mechanism it uses with a variety of metals (in particular copper and iron).