I like engine enamel from the automotive section of a superstore or an auto parts store. The advantage is that after it's cured, it is one of the toughest coatings in existance. The disadvantage is that it does not cure on its own. After you spray it, you have to let it dry for a day. Then it takes on a chaulky powdery look. If you rub it, it will come off. However, if you cook it after that with a heat gun, it gels up and hardens (cures). I've used it for engine parts that i've put into an old oven at 400F for a couple hours and with that kind of heating control it turns into a completely inpenetrable barrier despite the harsh environment of an engine compartment. This might not be practical for your situation, but there it is.