Supercab Rust

What causes the supercabs to rust along the bottom of the back portion below the rear (side) windows? Between the B pillar and the cab corner.

Is it the window? That was always my assumption but seeing how the gaskets are on the windows, that doesn't seem likely.

It also seems to occur an inch or so up.
For anyone that had done the repair, did anything stand out when you were in there?
Half tempted to take a sawzall to one in the jy and inspect it.

This is a place I am planning to spray the wax coating but got to wondering why it happens.

Edit: added some clarification

kramttocs wrote

What causes the supercabs to rust along the bottom of the back portion below the window?

I'm not familiar with Supercabs, but are you referring to the back wall, just above the floor, or "shelf" where the heads of the rear cab mount bolts are?

Is the area the same as a regular cab?

Pictures would definitely help this thread out - let me see if I can find some.

I am referring to the sides behind the door but not quite to the cab corner.
Around here, a lot of them have a long strip between the cab corner and the door that has rusted open.
Never having looked at one I am not sure if the channel inside (where floor meets side wall) is rusted or not.

Ahhh, OK. When you say back window you mean the back window on the side of the cab? My mind went to the back/rear window.

My fault. The question popped into mind as I was laying in bed so posted without much thought. Added some clarification in the first post but I really need to add a photo.

The spray foam holds water back there.

Never played with supercab windows, but if the caulk in the gutters is bad it will let water run down the B pillar.

The windows may have their own weak spot. I'm not sure.

This is what they end up looking like around here. This is a local truck for sale, not far from me. There are a lot of these style trucks sitting/rotting around here. Popular work/company trucks.

Do you think the door sill looks any better?    

Even the salty slush off your boots will run down there and sit.
Once you go uphill and it runs back under that foam it is trapped.

Cab corners rot out all the time....
Super cabs just have highly visible cab corners a foot long.  

So you think it's rotting from inside the cab out?

Hope he doesn't mind me pointing it out but Kevin's (Teach) truck shows some of this (northern truck). Maybe he can check the channel on the inside and see what it looks like.

I definitely do.

The corner does not have the upturned pinch weld between floor and door gasket flange.

Ford filled these valleys with foam to keep water from pooling there, but eventually water gets under and sits there unable to evaporate

Mine was due to storing a fire extinguisher behind the seat and at some point it leaked. Years latter rust showed up. I found a cab cornier from an 89 that worked on my 84. The other side is fine but it is a Southern truck.
For some resign I can't get the "insert image" to work. Maybe something on my end.
   

Is it telling you 'upload failed'

Any IMG or JPEG should work.
Click the 'large size' button to assure the file is not to big for the forum.

Often I have to wait for the pic to load in the reply window.
The server is slow....

Working now, before when you click on the icon it would not do anything.

Thanks guys. Since the rust is usually before the cab corner on supercabs I was trying to attribute it to something other than what causes reg cab cab corners to rust out.
I'll make a note to look inside the next supercab I see that has the rust showing.

It's a view I know all too well!  😢

Ill try and get a look and see what I can find on mine.

Off topic a bit but I really don't think Ford put a lot of effort in rust prevention of our trucks. Frankly, American vehicles from the early 70's to the mid 80's were horrible. Bad years for American automobiles.

Some of you know, I am the original owner of my 86 F150. This is a southern state truck with only 100k on it. I know where it has been! Thus far, I've changed out both rear quarter wheel lips and inner fenders. Last night, I completed the repair of both drip rails. My next venture will be to inspect the cowl and doors, as I'm now very nervous about where rust will develop next. Don't get me wrong, I love my truck but we got some problems in this genre.

If you're going to put a little oil in the bottom of the doors (to wick into the pinch weld) don't neglect to do the same to the tail gate.

It does seem to make a difference.

If I ever have one of these trucks painted again or looks don't matter like with my 1ton I would research getting some Eastwood Inner Frame coating (or similar) that comes with a wand and before sanding the truck down spray it in the:

Doors
Tailgate
Rear wheel wells
Cab corners (inside rear panel for supercabs)
Cargo light
Vertical runs of the cowl
Bed back corners
Bed support channels
etc

Some of these don't necessarily have to be done before painting but I used this stuff and considering how easily it runs, I'd be scared to risk it finding a way onto the new paint.

If it runs so easily perhaps it's thin enough to be drawn into the pinch welds by capillary action?