I used fiberglass mesh and fiberglass bondo a couple years ago to try to fix badly rusted cab corners (big holes). I didn't like dust from gravel roads and rodents getting into my cab. It basically worked just fine, except I had to patch a couple small holes just a couple weeks ago that had appeared at the bottom. I don't know if the patch gave way there, or new rust holes developed, or I didn't get it sealed very well there to begin with.

Bondo may improve things and I am satisfied, but those cab corners seem to be the way to go if you want to do things the "right" way.

 

Cargo lights with bad seals or cracked lenses are also where water can get in. Same goes for the roof clearence lights....

 

I had a water leak through the cowl, drivers side under the wiper linkage cover. The seam sealer had dried and cracked, and there was a small rust hole where decades of trapped pine needles lay wet. ~Bill

 

A few things that maybe go without saying, but still shouldn't be forgotten.

The first step in any rust repair is to remove ALL of the rusted metal. If you don't cut back to clean metal rust is going to come back right around the edge of your patch within a couple of years at best.

Trapping moisture against metal is sure to cause rust. A lot has been said here about finding and stopping where the water is coming in, and that's good. And comments have been made about debris holding moisture. But some "repairs" do too. Rags stuffed in to keep dust from coming in will accelerate rust (I know, that wasn't done as a rust repair). And spraying foam in will do the same thing. I'm not sure about fiberglass and/or Bondo. I could imagine that either one (done right) could seal out water, but I don't know that to be true.

Stressing metal also makes it more likely to rust. Bending and welding are two ways to stress metal, so any rust repair that can be done without either of those is more likely to last. Adhesives are better than welding for this reason, where they can be used. I don't know that there's a decent way to glue new cab corners on though.

Bottom line is that any "cheap fix" is likely to be a short-term imrovement only. And Neil Young was right.

 

Bondo definitely holds water like a sponge.
It's the nature of the chalk/talc like filler that is used to give polyester resin "body" (thixotropy)

This is why I always use a marine purpose faring filler (epoxy, NOT polyester based) for "repair".
It adheres much better, flexes better and does not absorb moisture.

Sure, I use "Icing" for pinholes and such.
I don't have time for the epoxies to cure.
But ultra thin and absorbent coatings are saturated with whatever primer/sealant is used anyhow.