Posted below is an excerpt from a July cruising world article I found on the subject, see what happens when you get your magazines after a year without.
"Plastic or nonmetallic seacocks and through-hull fittings should be made of glass-reinforced polymers, such as Forespar's Marelon. Like brass, such plastics as PVC and nylon should never be used in raw-water applications.
Different kinds of seacocks require different kinds of maintenance
. For the kind that's flanged and has a ball valve, little is required in order to keep them operational, although they're liable to become fouled with growth. Their Teflon seals
and chrome-plated balls are self-lubricating; like all seacocks, they benefit from monthly exercise or cycling.
Nonmetallic seacocks benefit from periodic greasing, which requires the removal
of the hose to expose the ball. Clean the ball, dry it, and coat it with marine
grease. My preference for the lubrication of all seacocks except those that use rubber balls or cones is Lubrimatic's Marine
Bearing Grease (www.plews-edelmann.com);
this lubricant is water-insoluble, sticky, and, unlike silicone, lithium, and other "light" greases, it resists washing
off. Rubber cone valves call for a lubricant, such as silicone, that isn't petroleum based.
More traditional cone-style bronze seacocks can be disassembled and lubricated for many years of service
. Do this annually; certainly do it every three years. Take the seacock apart, clean it with a solvent such as mineral spirits, and sand it with emery cloth to remove any irregularities. Then lap it using valve-grinding compound, apply lubrication, and reassemble.
Some seacocks can be temporarily fit with grease, or zerk, fittings, which can be installed in the drain plugs. With the seacock open, grease is pumped into the cone area. Because most zerk fittings are made of mild steel
(or sometimes stainless steel), they shouldn't be left in place. Make certain that the drain plug isn't corroded before it's reinstalled. If a new plug is needed, make sure it's bronze."