- What can a weekend mechanic do about it?

 

Without knowing (or seeing) how bad it is, the answer is not so easy. There are things like "rust reversers", but you still need to sand it smooth, primer it and paint it. Still, there is no guaranty that it won't come back.
Only a professional job can give you any sort of confidence that the situation is taken care of.

 

The rust is developing along the wheel well lip on the rear passenger side of the car. I tried to fix it myself once, then again professionally; but now it's back as bad as ever.

 

If this is the case, the rust was not totally removed the first time. It looks as though the best way is the replace the whole piece. It sounds like any "repair" (as opposed to replace) you do now will not be permanent. It'll eventually come back.

 

Doesnt honda have a lifetime rust-through warrenty? Where if anything rusts completely through they'll give a new part or did they just introduce this recently?

 

I'm not so sure about the "lifetime" thing. Most rust through warrantees have some sort of time limitation - 10 years? And if it has been repaired elsewhere, that part won't be covered anymore.

 

5 year unlimited miles rust THROUGH warranty. I just had my fender replaced and my car repainted...
Neal

 

I never even hoped that a 14 year old car with 211,000 miles would have any sort of warranty left...except maybe the automatic seatbelts.

The section that's rusting is part of the unibody...it's not exactly easy to just "replace." What happens in this instance? Do professionals cut out the cancer and weld in a new panel?

I've used a body shop in town that's expensive but VERY professional(they perform dealer work for BMW, Mercedes, etc.) for minor accidents in the past, and they refuse to work on rust. It's disheartening, but I still have hope that maybe I can learn how to do it myself and perhaps refinish it myself every year or so.

 

Rust in a unibody is a tough one. The body guy should have been able to advise you better than most people here.
I suppose you could search the internet for ways to do rust reparation on your own and since you are willing to attend to it as often as need be, you may have a better handle on it than a body shop.