Always learning something new. I have been getting these terrible surge marks on my B&W pictures. Regardless of how and how much I agitate or with pre-soak and without it, the demons appeared in the pictures. So, I tried to get help at Flickr, asking wheather there is a working recipe to avoid these marks.
Among the answers, I tried to follow the advice of one of them, coming from Larry D., a long acquaintance at Flickr. He insisted with me to refix the negatives and see. I did it and the surge marks were gone. The lower pic has the marks on the top side and the upper one is the result of refixing/washing.
Well, I learned that rapid fixer is not that rapid, it needs about 5 minutes to clear completely, at least with the film I have been using, Polypan F, and so, the fixing time should be 10 minutes using the rule of doubling the clear time.
My thanks to Larry D.!