most factory gas tanks are made of steel and I don't think they are coated with anything. I think because the tank is always full of fuel it doesn't rust (didn't they used to make steel gas cans?)
It's when it sits for a while when you have a problem.
I would suggest sealing it with POR15 or Eastwood sealer. It is better to be safe then sorry plus it seals any pinholes and seams.
t
POR15 makes a gas tank sealant, once you weld up your tank, grab a pint of that stuff, and swish it around inside to coat it from rust, and pinholes. It works quite well.