Here in the Northeast, rust is a constant enemy. It seems that rust forms in predictable places...on all cars....and Subarus are no exception. I've fought the slow progression on my 96 Legacy wagon for many years and no amount of rust converter, tiger hair, bondo, undercoating, etc seems to do anything to stop the spread.

 

After watching this progression on different cars, I'm starting to suspect that the root cause may be less obvious than salt on the roads. Certainly salt is a factor...all the under-side is exposed. Salt kills the suspension, brakes, lines, fuel tank, etc...but what about the body panels?

 

For years, I've assumed that salty water gets in to the fender at the lip where the inner and outer parts are crimped and welded. From there it migrates to the lowest spot and rusts from the inside. Well maybe that's so, but I've gone to great lengths to seal this area. Lately I've been wondering if the moisture doesn't originate from condensation.

 

In the winter, the inside of the car has warm moist air which can condense against the cold body panels. There is no way for it to drain from the inner fenders....it may eventually evaporate...or not. This would explain a lot about the way the corrosion moves.

 

If this is the actual mechanism, would it make sense to fill the cavities with expanding foam? No air = no condensation. What do you all think?

I can understand your logic using expanded foam. However, treating the metal with chemicals before body assembly is more practical. I have owned two Audi 5000s from the 1980's, and they simply don't rust anywhere. Pulling off a door panel, reveals massive rust proofing that is not found in Subarus. At the time, I lived in the Cleveland, OH area, and they use plenty of salt on the roads there. I don't know what Audi uses, but they found a way to make cars totally rust proof.

Another reason why the front fenders tend to rust away behind the wheel and in front of the front door is the filter/grate thing isn't quite fine enough. So small leaves like from a Russian olive tree will fall through it and clog up the drain holes so water tends to sit there and eventually that part of the fend will rust out.

Here in eastern Canada it's very popular to have your car "shot" with oil. It's a very messy process but it works.

Basically, you heat up a mix of motor oil and parafin wax,(everyone has a different recipe....some use chainsaw chain oil), and spray it on using a spray gun.....with a long nozzle.

Between panels where there are no factory holes or plugs,(such as inside the rocker panels), you drill a small hole,(about 1/4), and insert the spray nozzle.

Normally, the entire vehicle is sprayed,(where you can't see it of course). The entire underneath, between the panels, inside the doors and in the engine bay.

It's best to do it every couple of years.

 

It works but like I said it's messy...it drips out everywhere, especially in the summer on hot days.......at least you can wash it off....the rust you can't.

I would agree that if Subaru really wants to make an all around more durable car then they need to have much better rust proofing. I have a 2001 OBW that has quite a bit of rust. My daughters 1999 VW Beetle has only a tiny fraction of the rust I have. I guess it remains to be seen if the newer model years (ie 2005 on up) hold up better.