These people sell black liquid neoprene rubber in a 4-oz brush-on can. I've ordered some for bellows pinholes, but haven't tried it yet. It's actually manufactured by the Plastidip people and re-labeled by these dive shop folks as "Trident Neo Coat" (found that out by reading the MSDS sheets). Plastidip sells the same product but calls it "Liquid Tape". It even comes in a spray can (from Plastidip). Be careful about ordering liquid tapes and such, many are just a plastic vice rubber compound and won't be as flexible or last as long. This stuff is supposedly liquid neoprene rubber.