All you need is a waterproof coating material. There are a wide variety of products which work well: at your home improvement store there are lots of options such as urethane waterproof coatings, rubberized paints, etc. from an electronics supply story you can find silicone and urethane conformal coatings which also work very well. these coatings soak in well, stick to metal, and dry to a rubbery finish. if you are in pinch, you can do quite well just using standard oil-based paint, or nailpolish. these soak in and stick well, but are a bit hard when they dry so i suspect they may chip off over time (although i have not tried them enough to say for sure).