Chassis Corrosion - Cause and long-term fix

I am needing to tackle the corrosion on the rear chassis area. I realise its a very common area to rot out. But looking at the problem I cant help think that the cause is likely due to moisture settling internally in the low point in the chassis rail. With the open rear end of the rail tube along with its overlapping seam construction it seems very plausible the issue emanates from the inside. Has anyone got any informed views on this? If this theory is true then it would be logical to close/blank off the tube and/or add a drain hole at the lowest point.

Drill some more drain holes.
Use cavity wax and coatings of your choice every year.

All the lr box section chassis fail from the inside, there are a quite a few moisture traps in them.

Isn't this the entry hole for the moisture? Seems like its an uphill battle if this is not closed up. You often see people pathing this corrosion area up without adding drain holes or closing off the end of the tube. If anything that will act as a renewed water retainer.

Pointless trying to stop water getting in now or ever really. Where your corrosion is, and everyone else’s for that matter, is two or three layers thick. It’s a crap design, end of.

Many years ago I had the idea of putting old engine oil in the chassis. So set about counting how many holes I needed to fill to keep it in there, if my memory is correct there are 64 of them, so you have no hope of keeping the road spray etc. out of the chassis.

Anaconda said:

Drill some more drain holes.
Use cavity wax and coatings of your choice every year.

Click to expand...

Second this!
After you've had the rot cut out and repaired properly.
Air which is 10 % water at almost all times, condenses and then runs down to the lowest point. So this plus bare metal => rust.
Simple as.
The drain holes ought to make it run out faster, especially if it has been for a swim or a drive in the wet. Or has been cleaned with a pressure washer.
Best of luck!

With Hot/Cold cycles, air moisture will condense inside a sealed section, hence why car sills can rot out - will be same inside the chassis.
Not helped by baffles/strengthening plates welded inside the box section, trapping road dirt and/or mud keeping moisture or water around bare metal/welds.... just a matter of time before it breaks through from inside.
Waxes and paints dry out/flake off over time too, so oil is the best idea IMHO.
I like the ACF50 product as it adheres and coats nicely as a fine mist.
But engine oil / your preferred oil, even lanoguard, is OK, just needs reapplying every 12 or 24 months.

I mix some diesel with old engine oil and spray that around.
It's a tip and old boy told me back in the early 80's, and have done it with all of my keepers.
My Nissan Terrano was solid underneath, when I sold it, the buyer had owned them before and knew the places to check for rot.
He couldn't believe how solid it was.
I fire the gloop into doors and cills.
I try and avoid getting it on rubber components though