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What causes these crossmembers to rot? I know that the tool boxes rust due to water drainage through them but what makes the crossmembers deteriorate so often?

I'd like to do some preventative work on mine so I don't have the same problem 10 years from now.

Hi John,
If you mean the back bed crossmember, mine is the same way, and the one thing I noticed on all that I've seen is that it seems to be worse just right of the center tailgate mount. I have no idea why.
Gary

Yes, all three of my beds have rot in the same areas to differing degrees.

Both of my beds were full of cementatious, concretions of sand like stuff, I'm thinking beach sand as I did remove a small shell from my donor.
The donor was not under coated, the T/O had many layer of paint as well as the under coating, which saved my cross members & the rear valence.

I wonder how much trouble it would be to run tubes thru the tool boxes?
Not for me, but I'm thinking after it's all said & done, (repainted) after
fording, power wash from below should be SOP?

If you will notice, the vast majority of rust through damage on an M37 is either where two panels meet together or where there is a hollow cavity that water and debris can get into where it becones trapped. The debris holds moisture, and since these are always internal areas that is bare metal, as these areas couldn't be accessed for painting; they literally rust from the inside out over time. In the areas where 2 panels are back to back, under the headlights, corners of the cowl, 2 typical rust areas where 2 panels are sandwiched together, the issue is the same exact thing. Moisture and debris gets trapped, rust occurs inside out.

These issues are one reason a phosphate dip and powder coating are so effective in such areas. Submerging into a phosphate tank, allows the solution to reach most areas. This is a mild acid that neutralizes most trace rust left after blasting. Forced drying, powder coating and curing is the best way I know to handle this. The magnetic drawing of the powder onto and into cracks and crevices enables areas to be coated that could never be accessed with wet coating. Undercoating helps in some cases, but will crack over time letting moisture and debris get under the coating. It then causes the same inside out rust damage, thus the better solution is not undercoating.

I have a bed that someone along the way had drilled drain holes in the bottom of that channel so the moister had a way out they had done the same in the cab corners and this one is not rusted through. But VPW is now producing reproductions for M37’s.but if I were rebuilding it or installing new I would do like Charles mentioned to powder coat it and it will last forever.
hb