How does water get in Rocker panels?

I have been filling rocker panels and door pillars and even rusty doors with Big Gap Filler. I clean the areas and blow them out and then paint as much of the surface as I can with that heavy duty extra rusty Rustoleum Primer and then fill them with Big Gap filler. The stuff bonds very tightly to the metal and it can't ever flake off because the stuff expands and holds the material tight to the metal. If I have big gapping holes I make a form with waxpaper and then duct tape over that to keep the stuff from just falling out. I have found that with a little care you can get smooth paintable surface when you pull the wax paper off.

This is cheap and quick and flexible. Lot less work than bondo but more importantly the Gap filler keeps the metal dry and stops future rust.

So my question becomes why don't they just fill rocker panels with gap filler from the factory?

You dont want to use(Expanding foam) it holds moisture and will rot out very quickly.

Moisture has a way of getting into everywhere you don't want it. Like Sneaky Snake said, expanding foam will only make the problem worse by trapping moisture against the metal. Rocker panels are particularly problematic because of the way the panels are spot welded together - even if they painted the metal before spot welding, it will still get burned off between the pieces at the welds. Lack of proper drain holes can be an issue, plus they can get plugged. Add to all this is road salt in northern areas and the simple fact that spray from the tires in the rain is blasting right on the rockers. The best cure is to move to southern Arizona....

I've used that stuff to fill rockers and cab supports to keep the water out.
Mine actually last longer than just letting the salty snow pack up in there till spring.

I blow them out with a power washer and ez off the whole area to get the grease off then duct tape over the holes fillem up saw off the excess hit it with the grinder and cover the whole thing with kitty hair then roofing tar.
these of course are not trucks I'm planning on ever restoring just trying to get them through a few more winters before Mr Mayhem comes for them.
one truck i did I saw 10 years later and the rockers and cab supports were still intact. the guy had just kept up the fiberglass when it needed it.

I think the issue of attracting water depends upon if its closed cell or open cell foam. Gap filler is closed cell.

I have done this to about 10 vehicles and it worked way better than expected.

The japanese unibody racers do this to add rigidity to the body and the kids with the rediculous sound systems do it to make the sound better?

What I like is you can get the gap filler into areas that you would never get acess to with bondo.

I agree this is applicable to the daily driver and not a resto but hey every dollar you save on the daily driver is a dollar you can spend on your toys.

I suspect that putting this stuff in the front of the rocker panel is a great idea because my experience has shown that the front of the rocker pannel rust first because debris builds up behind the front fender well and when that area gets a rust hole the rocker panel will get filled with salty water and then the big dance is over.

it's either that or cold water up your pant leg in january

Does anyone remeber

Ziebart?

Sure do

That and most other internal undercoatings flake off and then they plug the drain holes.

Thats the advantage I see in the Gap Filler. It expands and sticks even to rusty metal although I still think its best to try to precoat with Rustoleum primer.

condensation is a mother effer