Cab corner rust... Why?

[firefighter]

I know the cab corner is one of the spots our Chevy trucks commonly rust. I'm not sure I totally understand why though.
I've talked to a couple body shop owners and they tell me most rust occurs from the inside out.

I can understand why rockers rust because moisture, dirt, mud leaves etc... can get into the rocker (since it's open at the back) and hold moisture for long durations, thus causing rust.
On the cab corner, it's totally sealed up, except for the removeable plug on the inner cab corner.
How does moisture get in there to start the rust process? At least I'm assuming it's moisture that causing it, right?
Is this a spot any of you guys clean out on a yearly basis or so?

I've never really thought much about this until my current rebuild. I sprayed as much undercoating down into that corner cavity as I could before I painted the inside of my cab. I figured that keeping moisture from reaching any bare metal was the important part.

Your thoughts on this please?

[Jason S]

Since other truck makes and other GM truck bodystyles suffer from the same issue, maybe it's simply a lack of paint protection combined with condensation.

[velojym]

Cab corner rust... Why?

...seemed like a good idea at the time?

[bake74]

     AH, another mystery question of the universe.  At least for me that is. 

[firefighter]

Am I to think I stumped the experts?  Naw...... this can't be true.

Usually I'm asking something dumb like how do I know the difference between a C or K truck, or how do I create world peace or something like that.
This must have been a decent question.   

I can really understand the rust in wheelwells and rockers, but the cab corner rust has me a bit perplexed.

[bake74]

     Maybe it's just where moister likes to collect inside the cab.

[fitz]

Rust is bad.
When I was bad as a kid I was sent to the corner.
just a thought..............

[jaredts]

Rust is bad.
When I was bad as a kid I was sent to the corner.
just a thought..............

Oh, Fitz...LOL.  At night it gets cold and the water drops out of the air in the cab and settles there.  If you hit a big puddle and water gets past the plug water settles there.  No matter how it gets there, moisture collects and can't get out.  I have to question if they should have left it open to drain instead of sealing that seam up.  Same with the rocker to inner rocker pinch welds.

[bake74]

Rust is bad.
When I was bad as a kid I was sent to the corner.
just a thought..............

     Now there is a man sharp on his toes today.   

[Donut]

You've got a lip hanging down, it's going to collect whatever crud is blown off of the front tires.  Removable plug?  I've only seen them in magazines.

You've got a metal on metal joint exposed to the elements, interior and exterior.  No easy way to clean it.  look how many leaky/rusty cowls there are, and that seam is somewhat protected and sealed.  Don't forget, these trucks only had to last until the warranty was up.

I currently have a southern truck.  Rockers are mint, cab corners are rusted and so is the cowl.

[firefighter]

Great reply fitz!!               That does make good sense.  Ha Ha !!

Some good thoughts and theories about why this happens. I just want to be proactive and not let this happen to my new cab.

Any other replies are welcomed and encouraged.   

[thirsty]

I think it's caused by the front tires throwing water, mud, and salt at the seams on the corners forcing moisture up in there. Then it doesn't dry out quickly enough. I sealed all the joints on mine hoping to keep water out. I also plan on keeping the inside of it coated with fluid film the best I can.

[Jason S]

I still stand by the condensation theory.

I've seen southern trucks of all makes that have spent their lives out of the salt, snow, etc. and still will get rust in the cab corners.  Granted the rust isn't nearly as bad as what I've witnessed from photographs on this site posted by the northerners, but it's a rusted through cab corner just the same.