Fan belts can pick up dust and debris through continual use. This dirt sticks to the friction-based surface of the fan belt and can cause serious problems over time, affecting not only the belt itself but also other components of the engine system that the fan is on. Serpentine belts can be notoriously difficult to clean, though, so when in doubt stick with the most simple cleaning procedures.
Most fan belts, especially those use in cars, are not designed to be cleaned, so there are few cleaners made specifically for the belts themselves. This means you should stick with the dependable, traditional cleaning methods, like brushing or, if liquid is involved, using simple soapy water. Sometimes during other types of maintenance, drops of oil or other substances can fall on the fan belt. These can be cleaned with the water solution, as long as you rinse the belt with cold, clean water afterward.
Many professionals use brake cleaner to clean serpentine or fan belts. The brake cleaner does not actually protect or seal the belt, however, and should be thoroughly rinsed off before the fan is used again. Some people actually heat the brake cleaner off after it has been applied to make sure all of it has been removed. There are also some companies that sell fan belt solutions that are designed to improve the life of your fan belt, but these are rare and their efficacy is debatable.
Make sure you remove the fan belt before thoroughly cleaning it--for light cleanings or brushings, the fan can usually stay on. It may be necessary to consult a professional or ask for help in order to remove and replace the belt correctly. Always be sure the belt is completely dry before putting it back on.
Keep in mind that fan belts are designed to be replaced, not cleaned. It may be that you need to just replace the entire belt, and no amount of cleaning will make a worn belt perform better. When you take the belt out for cleaning, use this as an opportunity to look for cracks, scratches or signs of wear in the belt. You should also keep in mind that many fan-belt problems originate in the motor or the bearings, not the belt itself.
Never use any sort of oil or lubricant on the belt. Fan belts need friction to operate correctly, and using a lubricant will only impede the operation of the belt. If the lubricant coats other parts of the motor, the whole system will need to be cleaned, which can be difficult and time-consuming.
