I have repaired pinholes in bellows using "liquid rubber" products such as Plasti-Dip and Liquid Electrical Tape and it worked fine. I have not tried it on shutter curtains but I would try it if I had a shutter curtain with pinholes. I have read several comments online from people who have used these types of products on shutter curtains who reported they worked fine.
BTW, I am aware that shutter curtains and bellows are not the same thing so there is no need to inform me of that fact. Thanks.
I used them to fix about 100 pinholes in my Graflex Anniversary Speed Graphic's focal plane shutter. It worked really well, eventually.
That fabric has two sides. The outside, facing the film, is coated in some kind of rubber. Applying the liquid vinyl (I used Plastidip) to that side didn't hold for very long. However, applying it to the inside, the side that faces the lens, worked very well and has held up for years. That side of the shutter is just fabric. I also coated the fabric, both sides, in lemon pledge, which made the rubber and fabric more supple and less brittle. It took a week to apply, because I'd have to do it in sections, let it soak, and then wipe off the excess. Overall, it was very time consuming, but it did work without issue once completed.
Whether or not that will work will likely depend on the fabric you're applying it too. I discovered with bellows, the Plastidip works well on small spots. However, if you have to cover large spots, then the liquid vinyl comes in contact with itself when you fold up the bellows, and it wants to stick to itself, especially if the camera gets hot. So I had to replace the bellows on that camera. With the focal plane shutter, it rolls up, instead of accordion folds, so you don't have to worry about the vinyl coming in contact with other vinyl patches, so long as you apply it to only one side.
I've seen replacement shutter fabric for sale, but it's never long enough for a 4x5 press camera, and it's not cheap. I've also heard of people making their own using fine weave fabric and impregnating it with liquid vinyl. Here's a link to an article about a guy doing that:
http://licm.org.uk/livingImage/Project06.html