Its common for creamers to be used to go on top of rust, from what I've read. And other than that, I'm not entirely certain what else you could do to coat the inside. You can just let it go, but if it was rusted before it'll rust again.
I would coat it, but since you're doing it you can make sure its done properly, not half-assed like the previous owner.
Good luck!
Turbos, Hondas, 4-bangers, what could go wrong?
Shiny: [...] Considering the weather you've had to put up with I'd say you get an Iron Butt award and a Frozen Nipple trophy to go along with it. First time I've ever posted the word nipple... it ends here.