I had that white speckling problem with a few sheets I had bought of Platine. I'd never used it before and always heard it was problematic. I received several bits of advice from different people and made good prints on by doing ALL the suggestions together. They went as follows:
1) prebrush the paper with distilled water
2) brush coat (I was using a rod)
3) use PO developer at 150º F.
4) let the paper develop for at least 5 minutes (during that time I saw a few white spots and they disappeared)
Maybe just one of those steps did the trick, but I was getting frustrated and wanted to see a good print on Platine. At that time I was a Crane's user. I'm hoping it will come back. The prints i made were quite good.
Do you do anything special when using that paper as a rule?