I used bondo to fix the rusted out areas of my truck. The usual areas, above all the fenders and behind the shield of the running board( right in front of the bottom of the door). I just noticed after months of the work being done that the bodywork near the running board shield cracked. It looks like the screws rusted out or something so it was just the bond holding it. Also, I noticed that there is some rust bleeding through in some other spots. The repaired areas are not that badly rusted so i thought bondo would fix it. Any advice? I am able to do most body work myself and would consider anyhting.

 

You are gonna have to strip those areas down to bare metal, and put a rust preventor on, such as POR-15, redo the body work, prime it, and paint it. Whenever you have rust, you have to put POR-15, or the like on it so the rust won't come back. Also, try to weld the metal so you don't have to use so much bondo, cause when you have that much bondo, it is bound to crack.

 

<cr>
As you have learned, filler (aka Bondo) does not deal with rust. There are a couple of other features you should take into consideration.

Filler is not waterproof. It will soak up water, which of course, leads to rust. In those areas where water may get to the filler from the backside, you need to apply a fiberglass-based filler, prior to applying the regular filler.

Filler is designed to be applied no more than a 1/8" thick. That means you have to work your metal back in shape before you start doing filler work.

Grind out all your old filler, getting to clean metal. Remove all the rust you can.

Apply a rust converter (i.e. Metal Ready by the Por-15 people). The converter will take care of any micro-rust you couldn't remove from the pores of the metal.

Since ordinary rattle can primer is not waterproof, you need to apply something that is. Epoxy primer is the way to go. It seals out water and seals in any micro-rust that was not converted.

After the epoxy primer has set up, scratch it with some 80 grit sandpaper. Don't sand down to the metal. The idea is to just give the filler something to grab to.

Start you filler work.

 

Ill try stripping them down. Im am worried about the area behind the shield, because that is the worst. Above the fenders, there is nothing wrong eccept or a few spots. I have POR 15, but didnt think to use it. I used the spray can rust prevenative. Does expanding foam do anything? Ive heard that it adds support, dosnt absorb water and helps keep bondo from cracking. Is this true?

 

<cr>
I can't envision the area you are talking about. Pictures would help if you have them.

Foam would add support but to what? If metal is rusted away, it needs to be fixed with metal. My other concern is the water. While foam may not absorb water, it could keep water trapped, with no way for it to evaporate.

 

The area I am talking about is the lower rear area of the front fender. I will put a picture up soon.I was wondering if the foam would add support to the bondoed areas.