For small-medium pinholes, I use Plasi-Dip, which is a liquid latex, flat black paint that can be found at any Home Depot or Loels store. You can apply it with a qtip on the inside and outside of your bellows. Works very well and last long.

You can also use gaffers tape on the outside for larger areas.

Ryan McIntosh
www.RyanMcIntosh.net