Black fabric paint from the craft section of the local Walmart is an excellent bellows patching compound, and it's opaque. The paint is used for making custom designs on T-shirts. It's easy to check a lens for fogging. Open the camera, open the back, set the aperture wide open and hold the shutter open on 'B'. Hold the camera up and look through the lens at a light source. Any fogging will be very noticeable. Be prepared to take the lens apart to clean the inner surfaces.
I bought a couple of old pre-1940 folders, a Zeiss Ikon Nettar and a Voigtlander Bessa. They both had lenses very badly fogged on the inner surfaces.