Cinema film has a carbon remjet backing. If it is to be shot as stills the remjet usually has to removed in a home darkroom before processing. What are the best ways to do this?
The Rem-Jet coat is a dispersion of lamp black (soot) in a binder of cellulous acetate phthalate. This binder is an “acid plastic”. It can be softened and washed away using an alkaline solution. Machine processing uses a pre-bath to temporarilly harden the film so it better withstands transport in a fast moving film processor. The pre-bath softens the Rem-Jet and spinning rollers, much like paint rollers scrub off the Rem-Jet.
You can make an alkaline solution and hand buff with a well washed “T” shirt. This can be performed the film has been processed but before drying. Water 27 to 38°C (80 to 100°F) 800 mL 800 mL Borax solution 15 g/L use 20g Sodium Sulfate (Anhydrous) 100 g Sodium Hydroxide 1.0 g Water to make 1 L
The Rem-Jet serves to protect the film from exposure from the rear. Many motion picture cameras with thru-the-lens viewfinders leaked light if the photographer looked away. It serves as an anti-halation backing. It protects the film when large rolls are loaded and unloaded in subdued light.
In the ECN-2 process there is an alkaline bath applied at the beginning of the proces that releases the remjet coating which is afterwards removed before development. This is hard to replicate manually in complete darkness, because you don't have visual feedback if the layer was properly removed everywhere. But the developer is alkaline as well and releases the layer, too.
You can develop, bleach and fix the films with remjet and after final rinse remove what is left with a soft sponge under running water. Then rinse again and use stabilizer or wetting agent as usual. This will make your developer and other baths dirty, but it does not seem to have any impact on the prints.
You can get bulk ECN2 chemicals that are up to spec. Probably cheaper per quantity than using weak kitchen chemicals.
The problem with "manually" removing remjet is that it ruins the emulsion...with contains your latent image! All of the DIY advice and Youtube videos will result in ruined film. If whatever method your using recycles the wash, and doesn't filter out the washed away remjet particles--or if it doesn't protect the emulsion side of the film during the process--remjet particles will be embedded in your film.
Kodak states in their ECN2 processing guides (still available), that it takes approximately 30 psi of water to remove the anti-halation backing satisfactorily. But you can't use 30 psi on the emulsion side without risking damage. Even commercial ECN2 processor machines ruin frames here and there. Whatever method you think up, either low-pressure water or cool air needs to protect the emulsion side of the film.