I wouldn't trust just paints and liquid stitch. I use a very thin repair fabric which used to be made in Holland it's designed for repairing tears in suits etc and to be invisible. The black version is light-tight, self adhesive a nd very thin & flexible, I've been able to repair large shutter curtasins with it as well, all I have left now is some dark-brown material.
For a set of badly damaged bellows on a Speed Graphic I was able to patch inside and out and it's hard to spot where the repairs are. I have a variety of different black acrylic paints for finishing from matt black to gloss, there's a lot of variation.
I've a few cameras with extremely poor or missing bellows and the best option is to make replacements. I've a 9x12 camera with bellows that are wrong (look like they've been changed)- they don't allow full extension of the lens / focus rail so they'll be changed as well.
At the camera fair I go to ocassionally one seller has a box of OEM replacement bellows for many European cameras, I'd guess these are ex Camera Bellows (now Custom Bellows) stock and are inexpensive. Camera (Custom) Bellows made/make the bellows for most manufacturers in the West, millions for Kodak alone
This thread popped up just as I'm about to make bellows for my half plate Thornton Pickard Triple Imperial, and I pick up a whole plate camera next weekend that'll need new bellows as well, in addition I have parts to make another whole plate camera, just missing a front standard which I'll make.