Educate me: why all the rusty fenders?

[Mossbeard]

I've recently acquired my first truck; a '66 C20. I've been learning a lot here but I'm wondering what causes the rust patches I've seen on so many front fenders (lower edge behind wheel)? Is there a design flaw? More important, after repairing the rust, what can be done to prevent recurrence while still keeping the truck looking original?

[LostMy65]

There's a drain hole that wasn't made big enough to allow the silt that builds up in there from draining. Moisture and dirt gets trapped in there and it rusts away.
Some here have made the drain hole bigger.
One just needs to blast some water in there to flush it out from time to time.

[NEWFISHER]

Design flaw that traps moisture at the lower section. Same with the cab corners on GM trucks all the way into the late 70's. When you come across an original cab corner in good shape, it most likely has a 3/8 hole drilled in it to let the water run out. That was a mod done by an owner who was aware of the issue many years ago.

[Improbcat]

There is a bad design that traps water/dirt in the fender brace attached there and causes it to rot out.
Things that help are to drill a decent sized drain hole at the bottom of the brace and to seal the edges & top of the brace with seam sealer to keep the water/dirt out in the first place.

[Mossbeard]

Thanks everyone! That's exactly the info I was looking for. Very helpful.

[1963c-10]

Don't forget the rusty hoods, floor pans, rockers (inner & outer) and inner fenders.

[Mossbeard]

Originally Posted by 1963c-10

Don't forget the rusty hoods, floor pans, rockers (inner & outer) and inner fenders.

Yeah, I get a trickle of water from behind the dash on the passenger side when it rains. If I don't find the leak soon I'll have rusty floorboards.

[BR3W CITY]

Mine rusted there as well, but strangely it did so in a very concentrated manner. I actually had about a quart of sand in that area, which held the moisture and caused it just to rust there.

[1963c-10]

Originally Posted by Mossbeard

Yeah, I get a trickle of water from behind the dash on the passenger side when it rains. If I don't find the leak soon I'll have rusty floorboards.

Check the roof seal....and the firewall where it meets the cab. Also check the windshield gasket as sometimes they get loose and I've seen water come in there also.

[WhippinSaw]

I want to power wash my lower front fender after watching this youtube video on 1960 to 1966 front fender rust repair . It shows why the fender rust at the 7 minute marker https://www.youtube.com/watch?v=oydlEWgNths#t=293

[markeb01]

IMHO the drain hole is big enough, if nothing is allowed in to begin with. Unfortunately GM designed the fenders to collect debris and form a compost pile in the lower fenders until the sheet metals bursts open on the front side. The only way to prevent it in the future is to close all the openings into the fender brace. Here’s how I permanently fixed mine: