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Soak spark plugs that have stains and yellowing on the porcelain in Oxalic Acid (Wood Bleach). It will make the porcelain look like new and also take any rust off the body of the plug.
Paint the metal body of your spark plugs with Penetrol (it's clear) to protect them from rusting. Penetrol will also keep your head studs and nuts from rusting and brass parts from tarnishing.