Jorge is definately the one to ask on this subject but it sounds like you have the light on during your presoak and I think that would cause the problem. I use the safelight for about 3 or 4 sec only after the film has developed for 3/4 of the total time that I guesstimate for each particular sheet's requirement. After seeing how far along it is I guess again and give it another 1, 2 or whatever min and check again with the light for 3 or 4 sec.