I use engine block paint, holds up to high heat and grease. Looks awsome, no bubbling, spots, etc...
Have used Chrysler Blue and Chevy blue, the tank in my link below is chevy blue on a 55g fw. It has been on for about ayear and still looks great.
Recently put it on my sw tank and figured it should hold up any salt spray on the back with a little routine cleaning. I have searched all around for tank backing and this looks the best IMHO.