How is the tank condition on the inside? The coating may have been an epoxy primer, which likely is best not stripped off. I wouldn't do anything without first doing a de-rust with EvapORust and verifying no leaks from the inside out. If none and tank cleans up inside OK, then I'd go ahead and tackle the outside..... filler if needed, a good sanding with say 320 grit and a self-etching primer.
