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Technically, yes, you can clean spark plugs, however, in most cases it's not worth it. We do not recommend it for a number of reasons.
Ultimately, you won't get the same performance from a cleaned plug as from a new plug. Electricity discharges best from sharp edges. A used plug will have worn edges, and cleaning may contribute to the wearing.
If you insist on trying to clean anyways, the cleaning procedure below was supplied to us by an old motorhead:
NOTE: Insufficient cleaning of a spark plug may lead to its failure in a very short period of time. Clean the spark plug thoroughly to avoid problems later. Remember, if a plug is fouling, its usually a result of engine side factors or incorrect heat range selection. If, after cleaning, a plug shows unusual signs of wear or doesn't come clean, its time to replace it.