With any type of coating on bellows, it has to withstand extreme flexing in the folds and corners. I have used water soluable black dye intended for automobile upholstery, d(painted on with a brush) but still find this flakes off a bit in the corners., whereas this same dye on car vinyl upholstery is very durable. Perhaps the aerosol automobile upholstery dye would be even more flexible? Neither one of these would actually "seal" pinholes at the corners, and this has to be addressed with other products, widely discussed in other posts.