You should use an epoxy type paint - 2-part polyurethane is best. It is very expensive and has noxious vapours and you need to use it in well ventilated conditions. Buying from a boatyard is cripplingly expensive, but you need marine quality or it will fall off.
Brewers used to sell me the paint very reasonably priced in 2.5 litre and 5 litre quantities (two tins of paint and hardener/activator combining to the total quantity. They still deal with Bradite but would probably have to order the marine one for you.