A common problem faced when repairing old items is what to do about old, cracked, hardened or shrunken rubber. Rubber naturally disintegrates over time as changes occur in its molecular structure. The changes can be accelerated by exposure to ultraviolet light (as in sunlight), heat or cold. Both natural and artificial rubber are susceptible to chemical changes and the changes are, unfortunately, irreversible. However, since some rubber parts are irreplaceable, hobbyists and repair technicians have come up with some creative solutions to improve the appearance and extend the life of disintegrating rubber. Here are some hints.
If the main problem is the appearance of the rubber, not its functionality, you can clean and polish the item to restore its gleam. For instance, restore Bakelite jewelry and handles on old appliances by rinsing them with hydrogen peroxide and polishing with a light abrasive like toothpaste. Shine with a vegetable oil like almond oil.
Rubber trim on old cars may be rejuvenated with a tire dressing product like Back to Black or a rubber dye like Forever Black. Rubber car parts can also be soaked in clean power steering fluid (since the purpose of the fluid is to maintain the rubber seals in the system. Mixed results have been reported soaking antique car gaskets in pure ammonia (use a silicone gasket maker for critical components). Old door seals can be coated with black RTV gasket maker to provide a more attractive surface.
Grips on old sporting equipment can be soaked in warm water with a few drops of detergent to clean. Scrub the grip, then roughen its surface with fine grit sandpaper. Small rubber parts for toys such as airplanes and cars can be softened in boiling water to restore enough resiliency to stretch onto an axle.
If all else fails, a replacement rubber part can be molded usting a latex mold. Clean the original rubber part and create a rubber mold by painting it with multiple coats of latex mold material (available at artist supply stores). Carefully separate the mold from the original part and place it in a bed of sand for support. Fill the mold with a silicone room temperature vulcanizing (RTV) rubber, or a Polyurethane Casting Resin (also both available at artist supply stores).
Don't use cast rubber parts for mission critical repairs, such as engine block mounts, where safety may be compromised.