All right, we're talking about rocker panels here. If you have visible rust all along the panel you are pretty much in for replacing the whole panel. If it's just at the ends near the wheel wells, you may be able to get away with patches. If you are planning to keep the car, anti-rust paint and bondo are not going to do the trick. Problem is that if you see rust on the outside, there is bound to be a whole lot more of it on the inside. There are also strategically placed structural bits inside the rockers, and if they are rusted through, they should be repaired as well. Moisture finds its way into there in many different ways, from leaking window frames or openings in the doorposts to plain old condensation. Often a major culprit is the fact that the drain slots built into the lip that protrudes downward from the rocker panel get plugged up - sometimes, ironically, with anti-rust goop. If you pull the carpeting away on the inside, you'll see a series of access holes with plastic plugs in them. Pry them out, so you can look inside the rockers and get a better idea of what's going on. I know of people in Holland who swear by Waxoil. The name pretty well says what it is. I've yet to come across it on this side of the pond, but I'm sure the same stuff is available under a bunch of different trade names. I believe it's only effective, however, when used over sound metal with, maybe, a little surface rust. Some folks mix it with engine oil and/or diesel fuel to make it flow better into all the nooks and crannies. POR15 and ZeroRust are good rust treatment products, but when you are dealing with structural metal or sheet metal that shows, welding in new stuff is really the only way to go. There are some reasonably priced Italian-made MIG welders on the market that are perfectly suitable for amateur use, and welding can be fun once you get the hang of it. There is a web-site you might find interesting. I don't have the URL on hand at the moment, but if you Google for Huib Wassink, you'll get to it. I wish I could be more encouraging and point you to an effective, cheap solution, but there just ain't no such thing :-(
Bob S.