how to stop inside of tailgate rust ?

[cknfrmr]

I see there are tons of products out there to stop and convert rust on metal. My question is. What or how to apply it to the inside of a tailgate? I can do all things under the sun to the outside to slow it down or transform it on a used/previous owned tailgate but my problem is its not gonna do me any good if I cant treat the inside. I need any suggestions I can get on how to pull this off or if it is even possible. And once again THANKS for helping me out.