Ok I developed the prints : I began with a bad frame which contrast was really low, but it was probably underexposed. Then I tried another two which were perfect. Seems like the processing is ok. Still have to get the exposure correct...I wonder if the problem isn't more the exposure. But I guess it's better to have no white spots (transparent spots) and high density rather than many white spots and low density...

Edit : forgot to say that the two good prints were made at grade 2 and 3. I see it depends a lot on the light and exposure.