You should try water-based resist, which is used in silkscreening and batik. It's made to paint of draw on fabric, sets up as durable latex, can be thinned with water, and comes in opaque colors, including black. I've used it on the corners of my Retina IIa bellows which can't take the added thickness of even masking tape and still close. I apply it on the outside and carefully dust with talcum after it has dried, since the bellows is clamped so tightly when closed and could stick when closed for a long period.
Any decent art supply store should have resist in 2 oz. bottles.