Steve, if you have repairs to make, do those first. You must use a spot area cleaner first to remove any dirt or oily film in the area of the repairs. Isopropyl (rubbing) or denatured alcohol works best for this. Once the area is clean, the treatment can be applied. Several threads exist here as to good treatments, but for wear or small tearing at the corners of the pleats, Plasti-Dip or liquid neoprene works best and is most resilient. Problems with the long folds of the pleats require more agressive measures, such as restabilization or the interior or exterior surface. This is dependent upon the material the bellows is made of.
For cleaning and improving the appearance of synthetic materials, a spray furniture treatment such as Pledge applied lightly works. Clean gently prior to the finish coat in the folds with a Qtip. Grime can be removed as stated with alcohol. Be very careful on older "saturated" fabric materials, as the cleaning or finish application can actually dissolve or weaken the black or red coating that is impregnated in the fabric cover. Polishes work best with smooth vinyl coverings.
Leather is a special case. Once repairs are accomplished, a good liquid conditioner such as Kiwi Leather Lotion is recommended. The greatest cause of problems in leather bellows is the drying out of the material, and subsequent cracking, feathering, and flakeing. Leather lotion cleans and conditions the material, adding needed oils or moisture to the leather material. This helps prevent the aforementioned problems. Apply with a soft bristle brush, and then buff gently in a few minutes with another, clean soft bristle brush such as a 2" paint brush. The same sort of brushes are recommended for applying and finishing synthetic surfaces.
