Covering it in paint only stops you from seeing it. It's happening as you sleep underneath the rust you can see where the real metal still exists whether its painted or not. But if it is just surface rust it won't get that much worse in the next year and a half and that's about all the you could expect to get out of covering it without prepping. Don't fall a rust inhibitor if you plan on restoring it later. (Unless your paying someone else to strip it) Drive it till you can do it right.
I agree with CASO. The so called "rust converters" that spray or paint on and claim to convert rust to an iron oxide primer are a waste of money IMHO. They merely cover the rust with a layer of primer but the rust continues underneath.
However,
I am a firm believer in rust encapsulators like POR15 and ZeroRust which seal out moisture and prevent future rust. Don't apply these as a "time buying" step. They are expensive. Use them when you restore the truck properly.
It sounds like you merely want to buy some time and SLOW DOWN the rust process until you can address it properly. Paint will only diminish the amount of moisture that gets through to the metal, but it won't stop the rust. Also, water will penetrate most primers, (except epoxy primers), so regular primers won't do the job either. Epoxy primer is something to apply to properly treated metal that has had the rust removed.
So I guess I'd recommend to keep it away from rain and water if possible and treat the rust properly ASAP. If you have thinning paint, I guess you could add paint to that, but it seems a waste of money if you need to turn around and treat the panel properly anytime soon.
