Again why seal it? You go from one extreme to another. First not going to paint it at all, not applying sealing coats. Its paint, its already sealed. Its also in you closet in you bedroom. If it gets so wet that sealing it is the only way to protect it, you have much bigger problems than the paint on the back of the tank...
I've never done anymore than just paint it, as i've stated before, and it works just fine. Some of those tanks are over 4 years old without an issue.
pi