Fully extend the bellows and paint on the bad spots - MAKE SURE THE STUFF IS DRY BEFORE YOU COLLAPSE THE BELLOWS OR YOU'LL BE CUTTING THE BELLOWS FOLDS APART.
The newer the material, the thinner it will be - you'll probably need more than one coating.
Both, I believe, are a neoprene suspension (if memory serves me correctly, MEK is the solvent). If you go this route, make sure the solvent within doesn't do more damage than what you already have as MEK is pretty hot stuff. No problem with leather, but the naugy hides will probably be melted on contact as it's a plastic.