Do you have experience with long exposures on Ilfords BW negative films with strong ND filters (etc. Hitech 3,0 ND Pro Stop, Hitech standard ND 3,0, Lee Big Stopper, B+W 106 ND, B+W110 ND)? Long exposures in daytime for 1min., 2min. max 10 minutes.
I tried Ilford Pan F Plus with orange filter and Hitech 3,0 Pro Stop ND filter for this work. Ilford Pan F Plus, because is slow film ISO 50 and with Perceptol EI 25 an this film have poor reciprocity charakteristic. This reciprocity failure is benefit for this work. I rated Pan F Plus as EI 25 and developed with Perceptol 1+1.
Do you have any pictures with long exposure on Ilfords BW films? I would like to see some works and discuss about this point. I would like to discuss about good choice of strong ND filter for BW negative.
Have you tried using 2 polarising filters instead of ND filters? It may give you more control.