Has anyone here built an auxiliary fuel tank before?

I have been looking at prices for in-bed aux tanks. Sheesh! You would think they are made of gold! I mean, 16 gauge steel plate doesn’t cost that much! http://www.dieselplace.com/forum/smileys/Confused.gif

Anyway, would the inside need to be sealed with something or are they (steel tanks) shipped with paint on just the outside? I plan on baffeling the tank and adding a fuel level gauge.

Any suggestions? http://www.dieselplace.com/forum/smileys/Clown.gif