heaters shed that black enamel all the time.
heating, cooling, light exposure, and the water...all slowly degrade the black stuff and make it flake off.
i just wouldn't even mess with it. let it flake off.
if you have a canister filter, look into the Hydor inline heaters, and then you can completely remove the clutter of an in-aquarium heater...its really nice.
Former advisor and planted tank geek...life's moved on though.