Rust prevention for black or carbon steel

I am working on a project for an internship this summer. I am designing a dip tank for power transformers. The tank is 13 x 15 x 1" (LxWxH) and will hold a solventless polyester resin varnish. The varnish company recommends using black iron or stainless steel for the tanks, but said in my application, carbon or black steel would work because of the small volume. Their only concern was that the steel may rust and contaminate the varnish. I am not familiar with the different finishing processes available to prevent rusting.

We don't have enough money budgeted to go with stainless steel, so if there is a coating I could use, my project will have a better chance at getting approved.

Thank you for your help.

You seem to have lucked out on replies so I will suggest the one thing that I would do. Paint the tank with the varnish it is going to hold! This will not cause contamination issues and you should have the stuff to do it on hand.