How to Apply Body Filler and Spot Putty

These days cars are made with thinner materials than in the past and as a result, even a slight impact can cause damage to the body. Auto body filler is used to fill holes, dents, and deep scratches in the body of the vehicle.

Very often body filler is referred to as Bondo, which is really a product brand name manufactured by 3M. However, since 3M was one of the first companies to produce body filler, it became known as Bondo.

The quality of the product varies depending on the brand. Some are easier to sand, have better adhesion, and last longer but might cost a little more. Other cheaper products are harder to apply and can take hours of sanding. But no matter how much we dislike dealing with body fillers, there is no way around them if you are trying to fix auto body.

When buying a filler, make sure it has great adhesion, easy to sand, and spreads without a lot of pinholes. I used to buy Evercoat products but recently switched to Bondo Ultimate Body Filler by 3M. This is a professional quality product that is very easy to apply and will last for a long time. It also doesn’t clog the sandpaper.

After coming out, it got a lot of positive reviews from auto enthusiasts and body shop experts. It costs around $20 for a quart and comes with a cream hardener. This new product really works great on different types of surfaces.

You can apply it to bare metal, wood, aluminum, epoxy, sanded paint, and primer. Although I always recommend sanding to the bare metal, with this product you really don’t have to.

Preparing the Surface

Before applying Bondo, you need to prepare the surface. Wash the car with warm water and auto soap. Follow it with pre-solvent to remove all dirt and wax. Since fillers stick better to the bare metal, you need to remove all the paint by sanding down the damaged area with 180 grit sandpaper.

Don’t forget to sand about 3 inches around the damage for better blending. The more scratched up the metal is, the better Bondo will stick to it and the longer it will last. If you apply body filler directly to the paint, it will crack after a while.

Note: I’ve been getting a lot of emails from people asking if they need to spray the metal with primer before applying filler to prevent rust. You do not need to apply primer. Bondo is specially designed for bare metal and should prevent it from rusting. If you found rust, that means it was there before Bondo was applied.

Mixing Body Filler and Hardener

Bondo consists of different materials including resin and talc. It usually comes in a can with a cream hardener that needs to be mixed in to start the curing process. If there is no hardener in the package, you’ll need to buy it separately.

One of the biggest challenges when dealing with Bondo is to know how much hardener to mix in. That is where all beginners make mistakes. If you put too much, it will dry too fast and might create a lot of small pinholes. If you don’t put enough, it will take forever to dry.

Instructions in the back of the can should give you a good idea of how much hardener to use. But as a rule, you should add about a 1-inch strip of hardener to a golf ball-sized amount of filler.

After opening a can with a filler, check to see if it has separated. Give it a good stir to create a uniform color. With a putty knife (or you can use a regular knife) place some filler on the plastic mixing board.

Although you can use any clean surface for mixing, a plastic mixing board works best. A piece of cardboard will also work just fine. I’ve seen some discussions about using cardboard for mixing and some people were saying that cardboard will absorb chemicals from the filler reducing its effectiveness.

It might be an issue if you are doing a whole car, but if you are doing a small area, I think there is just not enough time for it to absorb that much.

While Bondo usually comes in a cream or light gray color, the hardener is deep red or blue. The color of the hardener will not affect the curing process.

To prevent filler from drying out, close the lid of the can after using it.

For mixing you can use a steal or plastic spreader. I like plastic spreader more because it makes application a little easier.

When pressing against the surface, a plastic spreader can be bent to the shape of the repaired area, something that you can’t do with steel.

Mix the products together by spreading and folding the mixture using back and forth motion and by pushing it down on the board to release the air.

If you stir it, the air will get in creating a lot of small pinholes. In the end, you should have a light pink or light blue consistent color with no streaks in it.

Note: You might also get pinholes from applying too much hardener.

Applying Body Filler

Once you’ve done mixing, you have only about 5 min before it hardens. So, work quickly! When applying the mixture, don’t put too much, otherwise, you’ll be sanding for hours.

However, you also don’t want to put too little and have to go back to mix another batch. Apply the mixture with a plastic spreader spreading a thin layer 3 inches beyond the damaged area. You need extra space for better edge blending when you sand.

Note: Avoid putting more than ¼ inch of filler. 1/8th inch is a good thickness. If you put too much, it might crack later. If you have a dent that is deeper than ¼ inch, try pulling it first to eliminate the amount of the Bondo that is needed.

If you have some mixture left that you are not going to use, don’t even think about putting it back into the can. Mixing old Bondo with the fresh one will ruin the whole can. Just throw it away.

Curing time depends on a few things like the temperature in the room, humidity levels, and the amount of hardener that was applied. On average it takes around 20 min. The ideal temperature for mixing Bondo is 70-80 degrees F.

Sanding Body Filler

After it is dry, you can start sanding with 40-grit, followed by 80-grit and 180-grit. Most of the filler around the damaged area should be sanded away leaving the dent filled.

Make sure the transition is smooth. It is better to do all the sanding by hand. You will have less control with an electrical sander.

Note: If you realize that you need to apply more layers of the mix, just sand the area and apply the next coat. In case you need to remove a cured filler, use an automotive paint stripper or just sand it down.

Applying Spot Putty

Spot putty is another type of filler that is used to fill tiny pinholes in the body filler. If you don’t fill those pinholes, the filler might crack when there is any movement of the underlying metal.

It is very easy to sand, dries fast, and doesn’t need a hardener to cure. You can just apply it from the tube. I usually use Glazing & Spot Putty by 3M.

Use a plastic spreader to put a thin coat of spot putty on the sanded area. Avoid putting too much.

Wait for 15 min for it to dry and sand it with 180 grit then follow it with 320 grit. The surface should be smooth and ready for primer. This is how it should look after the spot putty is sanded.

I know that applying Bondo is not the easiest thing in the world, so don’t hesitate to ask questions. I am always happy to hear from you.