The acrylic artist paint from Walmart etc. works as is noted above, it stays a bit soft and flexible but won't let even a little bit of the dark out of your bellows. Matte Medium mixed into a little bit of copy machine toner also will
work. The liquid electricians tape, and the acrylics will do a much better looking job than any of the Duct/Gaffer tapes. Can be applied both inside and out, you can even match the color of your bellows. Have been using the acrylics for 30 years or more without a problem.