It really depends on how bad the rust areas are how one would fix the situation. Minor surface rust can easily be sanded off using progressively finer grain until the surface is smooth enough that when you paint there will not be a seem. On a dry still, day paint the sanded areas with a nice hard spray paint like epoxy. A primer is not required as you say this will be a beater, but you could primer and then sand again before applying the final coat. Black is good because there will be no issues with matching the color.
As for the inside here is a god product, though I not sure about UK availabilityhttp://www.jensonusa.com/!CHINy0X3QZ60a ... erosol-Can
I was old school before it was cool