Yes, I have had problems with calcareous deposits too, even though rigorous washing and using demineralized water as the last bath of the rinse... Strangely, only about one in 3 prints showed spots (often a lot), while the others were often free of spots.

After an advise of Tim Rudman, I have now also routinely started using an acid (acetic) bath after the thiourea dip. This has resolved the issue completely.