I have been studying the rocker structure closely since I will be replacing a section of both rockers at the back. My first observation is that on my car there is almost no evidence of rust on the inside of the rockers, EXCEPT where the drain hose is.
The rocker itself is a beam composed of three different pieces. You can see that here in the RealOEM image:
There is an inner piece (9), which is what you would see inside the car with the carpets and insulation out. There is the zig zag beam (8), which I suspect is the primary structural element, and there is the rocker "cover" (7). "Cover" is an odd way to describe it because in the front and rear it is covered by the outer bodywork, and then there is the removable trim cover over that...
The structure is more complex at the "pillars". @Sven's phots are very illustrative! I hope he doesn't mind me reusing them with some detailed explanation!.
Here you can see that the A and B pillars and the rear wheel arch all attach to the zig zag member. These will ultimately reside UNDER the rocker cover, which has cutouts on the top that they rise through, forming the basis for the upper body.
Right side A-Pillar attached to the zig zag beam:
Left side B-Pillar and Rear Wheel Arch (lower C-Pillar) attached to the zig zag beam:
These structures are then covered by the "rocker cover" (the black component), which can be seen in place here:
The rocker cover is then itself covered at the front by the fenders, and from the B-pillar back by the rear quarter panels. The only part of this that is visible is at the doors (with the rocker trim removed).
What this means is at three places (front, middle and rear) there are FIVE layers of metal - Inner rocker, zig zag beam, pillar, rocker cover and fender or quarter panel. The space between these layers ranges from about 1 cm to zero. Here is a rough cross-section view of the B-Pillar area.
So, you can see why this is such problematic area! There are all sorts of layered joints (usually a haven for rust), and cervices that will catch and hold water.
Here are some views down inside the right side rear window area of my coupe. You can see the quarter panel, and the tops of the lower part of the B-Pillar and the top of the rocker cover, and you can see how the pieces all converge at the bottom. Of note in these images is that, at least on my coupe, there is relatively limited rust on the TOP of the rocker cover, and only minor rust down in the cracks, or the bottom of the B-Pillar. This implies that there is not much water intrusion from the window seals, other than in the last pic down inside the B-Pillar, where you can see that some water must leak down the spot at the front of the rear side window (where the side windows meet).
Now, compare this to the mess under the rocker where the drain goes... This is the outside, before I cut open the quarter panel.
And here is the same section opened up. The lower portion of the rocker cover is basically gone, and the lower C-pillar and zig zag beam are rusty but solid.
And a shot of the same area from below.
Bottom line is I am not super sure how you might get Waxoil into all the parts of this structure. Presumably drilling some holes in the inner rocker might work. that would get wax into the inner rocker area and behind the zig zag beam. To get the front of the zig zag beam, you would need to drill holes in the top of the rocker cover, and then to get the inside of the quarter panels you would probably just spray wax in from the rear window space and inside the kick panel. You might also consider drilling holes in the A-pillar and B-pillar and spraying inside those, although I am not sure much water gets in there unless you already have serious rust. For any holes that you might drill, I would fill them with a rubber plug of some sort.
I would also think that if the body has never been wax-oiled it would be better to start by spraying POR 15 or some rust converter into these spaces first, otherwise you are just putting wax over rust.
One other issue I have observed is that cars rust regardless of whether they are driven in the rain or washed because the moisture in the air condenses on the cold surfaces of the car, inside and out. The only way to truly avoid this is to keep the car inside a heated garage, or keep heater inside the car, so the metal is always above the dew point. My 635 has floor pan rust simply from being parked covered in my driveway over several winters. Seems that cavity wax would help a lot with this since the water will condense on the wax surface and not on the metal.
The rocker itself is a beam composed of three different pieces. You can see that here in the RealOEM image:
There is an inner piece (9), which is what you would see inside the car with the carpets and insulation out. There is the zig zag beam (8), which I suspect is the primary structural element, and there is the rocker "cover" (7). "Cover" is an odd way to describe it because in the front and rear it is covered by the outer bodywork, and then there is the removable trim cover over that...
The structure is more complex at the "pillars". @Sven's phots are very illustrative! I hope he doesn't mind me reusing them with some detailed explanation!.
Here you can see that the A and B pillars and the rear wheel arch all attach to the zig zag member. These will ultimately reside UNDER the rocker cover, which has cutouts on the top that they rise through, forming the basis for the upper body.
Right side A-Pillar attached to the zig zag beam:
Left side B-Pillar and Rear Wheel Arch (lower C-Pillar) attached to the zig zag beam:
These structures are then covered by the "rocker cover" (the black component), which can be seen in place here:
The rocker cover is then itself covered at the front by the fenders, and from the B-pillar back by the rear quarter panels. The only part of this that is visible is at the doors (with the rocker trim removed).
What this means is at three places (front, middle and rear) there are FIVE layers of metal - Inner rocker, zig zag beam, pillar, rocker cover and fender or quarter panel. The space between these layers ranges from about 1 cm to zero. Here is a rough cross-section view of the B-Pillar area.
So, you can see why this is such problematic area! There are all sorts of layered joints (usually a haven for rust), and cervices that will catch and hold water.
Here are some views down inside the right side rear window area of my coupe. You can see the quarter panel, and the tops of the lower part of the B-Pillar and the top of the rocker cover, and you can see how the pieces all converge at the bottom. Of note in these images is that, at least on my coupe, there is relatively limited rust on the TOP of the rocker cover, and only minor rust down in the cracks, or the bottom of the B-Pillar. This implies that there is not much water intrusion from the window seals, other than in the last pic down inside the B-Pillar, where you can see that some water must leak down the spot at the front of the rear side window (where the side windows meet).
Now, compare this to the mess under the rocker where the drain goes... This is the outside, before I cut open the quarter panel.
And here is the same section opened up. The lower portion of the rocker cover is basically gone, and the lower C-pillar and zig zag beam are rusty but solid.
And a shot of the same area from below.
Bottom line is I am not super sure how you might get Waxoil into all the parts of this structure. Presumably drilling some holes in the inner rocker might work. that would get wax into the inner rocker area and behind the zig zag beam. To get the front of the zig zag beam, you would need to drill holes in the top of the rocker cover, and then to get the inside of the quarter panels you would probably just spray wax in from the rear window space and inside the kick panel. You might also consider drilling holes in the A-pillar and B-pillar and spraying inside those, although I am not sure much water gets in there unless you already have serious rust. For any holes that you might drill, I would fill them with a rubber plug of some sort.
I would also think that if the body has never been wax-oiled it would be better to start by spraying POR 15 or some rust converter into these spaces first, otherwise you are just putting wax over rust.
One other issue I have observed is that cars rust regardless of whether they are driven in the rain or washed because the moisture in the air condenses on the cold surfaces of the car, inside and out. The only way to truly avoid this is to keep the car inside a heated garage, or keep heater inside the car, so the metal is always above the dew point. My 635 has floor pan rust simply from being parked covered in my driveway over several winters. Seems that cavity wax would help a lot with this since the water will condense on the wax surface and not on the metal.
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