Paint

Epoxy.
Make sure its marine safe.
Prepare to spend lots

I haven't found it anywhere locally, I'd recommend looking online.
Good luck!

I don't know how well it'll hold up on a heater, but here's some aquaculture-safe paint:

http://www.aquaticeco.com/index.cfm/fuseaction/product.detail/iid/11733/cid/3035


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I have used alot of epoxy (couple different kinds, also) on my current aquarium and when working on yachts. I WOULD NOT use an epoxy to coat a heater. With those titanium heaters they have external temperature probes, and the heater will have to heat to higher temps (how much is questionable) to get the same water temperature. You could have heater failure, possibly destroy, or even worse break-down, the epoxy with so much heat. Many things change chemical composition when exposed to extreme heat. I wouldn't play around with that in an aquarium. At the very least, contact the manufacturer and ask him.