Can You Use Expandable Insulating Foam As A Rust Hole Filler?

preist · May 21, 2008

I've cut a large rust section out of my fendor on the underside (hidden by the plastic flare). Instead of trying to form a sheet metal replacement piece what does everyone think about just using expandable insulating foam to fill the gap and keep water from being driven into the openning? I'll prime and paint the surfaces first...

Comments? Other ideas?

Nick The Great · May 21, 2008

My cousin did that on his old 78 3/4 Ton Chevy. Like a lot of 20+ year old trucks, it had serious rust issues on his rear fenders. It worked ok, but he tried to paint it, and it shriveled and fell off. He then got wise and put duct tape on it, and spray painted that! Worked real good for quite a while actually.

Coincidence or not, he'd always been known as "Bubba"

Don't know about your truck, but if it were mine, I'd want to do it right.

sam5828 · May 21, 2008

sure, just fill it then sand when dry, then either use bondo or fiberglass. then sand and paint wont be perfect but better than a big hole

preist · May 28, 2008

So I've cut away the main rust, sanded and prime sealed the rest. The expandable foam seems to have bonded well but I've applied metal foil tape over the openning to further resist driving moisture in. Here are a couple pics of the fender with cut away section.

I'll let you know if this strategy fails miserably.

pennzoil29 · May 28, 2008

I used it on an old '74 C20. I mainly just used it on the cab corners to keep the cold air out in the winter time. I didn't much care about the looks, as I beat the piss out of that thing.

kuhl_530 · January 22, 2009

haha I did the same thing with my 91 jimmy on the driver and passenger side door bottoms. Before I foamed it, I could reach up from outside the bottom of the door, grab onto the door release bar, and open the door, hahaha. The foam worked great at insulating it, but it looked like poop!