How dense is unprocessed film base

If I load 35mm or 4x5 film backwards, and expose it "normally", how many stops underexposed will it be? Is the base totally light-proof?

The base is not totally light proof. As a guess, probably 3 or 4 stops at least, and exactly how much depends on the particular film.
If this is film you've already shot, you may want to do some tests

I am wondering if it might be practical to use film base as an ND filter.

A ND filter would be more practical film would scatter rather than image and it would alter transmission over time.

BetterSense said:

I am wondering if it might be practical to use film base as an ND filter.

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No.

Reasons already cited plus your focus might be off, and the pressure plate (at least on 35mm) is probably going to scratch the emulsion.

It would also depend on the film, its age and the developer you choose. And the effect on 135 and 4x5 would probably be different too. No golden rule here.

BetterSense said:

I am wondering if it might be practical to use film base as an ND filter.

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You're better off making an ND filter from a developed and fixed piece of 4x5" film.

Differs significantly with different kinds of film. Not only does the emulsion itself cause a degree of neutral density, but the kind of antihalation coating involved acts like a colored filter, itself differing from film to film, with roll films deliberately having a stronger antihalation layer, then you've got minor differences between the actual types of film base plastics too. So it all depends.

It very much depends on the film, its age, how developed, etc.
For some old Tri-X I've shot, the unexposed base (fog) has been a lot more dense than the image in some (attempted/failed) underexposure/pushing in TMY. The RPX25 I just developed has got to be one of the cleanest / clearest bases I've ever seen, I reckon it'll take quite a few layers of that to affect exposure by much.

Also, placing any ND filter at the film plane is not a good idea:
a) if you jam anything between the opening and the film, it won't be in focus, may not fit, and could scratch the film. Same if you run the film backwards (but not as extreme).
b) the light rays are focussed at that point, any tiny scratch or dust on the filter will show up badly.
c) anything in the light path at that point will affect sharpness, for a primer read this about what happens when you put different thicknesses of glass near a digital sensor (putting film or an ND in front of film will do exactly the same thing).
d) it's a lot better to put it in front of the lens, the light rays are unfocussed at that point, so imperfections won't show up on the image. As said, some 4x5" film is better for that...

And it should be an exposed, developed and fixed bit of 4x5" as you wont be able to focus through an unprocessed sheet, it's not optically translucent so to speak.

From bitter experience with my first batch of Fomapan 100 4x5 sheet film. It exposes quite nicely through the back. I rate FP100 at EI 50 and an exposure through the back is only about 1 stop less exposure. Surprisingly sharpness is not much harmed either. Even a double load delivers a recognisable exposure to the bottom sheet. A sheet loaded with interleaving paper on top of it still delivers an image; blurry though.

The antihalation layer works as well as it does because it gets two bites at the light that gets through the sensitive emulsion; one bite on the way in and another on the way out. In backwards exposed film the AH layer gets only one bite.

Learning happens and I don't make those mistakes any more: fingers crossed.

When film is rolled backwards for "redscale" film, the exposure increase recommended is two to four stops. I have only used Rollei Redbird and Lomography Lobster which are already re-rolled and the ISO recommended. I would say, try some exposures and see what works.

bsdunek said:

When film is rolled backwards for "redscale" film, the exposure increase recommended is two to four stops.

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Colour negative does tend to have a rather brutal anti-halo layer, so the light loss would be rather high on a colour negative.