How To Use Body Filler

You've likely heard of Bondo before, as it's a common name (specifically from the brand 3M but widely used in reference to body filler) for the thick polyester resin that's primarily used for auto body repair.

When used properly, bondo- or body filler- is a perfect material for repairing dents, scratches, and other bits of damage to a vehicle quickly and seamlessly. But applying filler incorrectly can lead to an uneven body that will cost you even more to repair.

Do it right the first time by following this guide on how to apply bondo or a different body filler product.

What is Body Filler?

In general, body fillers consist of a polyester resin with a cream hardener that forms a putty-like substance that is used to return a vehicle's body to its smooth surface after minor damage.

Think of it like the spackle used in home repair to cover nails and even out walls. Their consistency and usage are similar, as well as the process that the surface it's applied to undergoes.

The difference with auto body filler is that there are three different types, and the type you use will depend on your restoration project and the results you're expecting.

The different types of body filler

Standard, medium, and premium grade are your three filler options, and the type you select is predetermined by the extent of automotive damage you are repairing.

While it's helpful to have all three options on hand, using the right one is essential for a proper finish.

Standard - lightweight putty that cures in approximately 20- 25 minutes. Standard grade fillers are typically only used on small scratches and minor dents such as hair damage.
Medium - also known as a fiberglass filler, medium grade fillers are useful for tiny holes or tears in either metal or fiberglass.
Premium- or aluminum filler, is called such due to microscopic aluminum particles in the consistency of the mix. The benefit of using this expensive filler for larger repairs and dents is that it doesn't shrink, is easier to sand, and is stronger than other fillers.

If you're looking for a particular primer, some of the most popular brands include:

3M (Bondo)
Dynatron
Evercoat
U-Pol
POR- 15
USC

Where to Use Auto Body Fillers

Body fillers have gained a bad reputation because so many people don't know how to apply body filler correctly- slathering on the mix too thickly without sanding or covering too large an area. Truth is, nearly every car that goes through body repair has filler on it.

Body filler is such a thick, easy to apply substance that some get a little overzealous with it. Another issue is that it's used in the wrong places.

Here are a few places that body filler should not be used:

Rust - putting filler over rust will only make things look worse.
Full gas tanks - body filler is flammable, and only resistant to gas once it is cured.
Anywhere that gets too hot - you'll simply end up with a clumpy, popcorn ceiling- like look.

If you want to use filler for areas with rust, sand off and corrosion or cut it out completely and batch with sheet metal or mesh before adding filler.

So, where should you use filler? Here are a few places:

On hail damage
Small to medium dents
Scratches that go below paint level
Small holes

It's very easy to mess up a small patch job, which is why it's best that experienced professionals or experienced DIY enthusiasts handle these types of repairs. It is possible to learn how to do these repairs yourself, but you should at least have minimal experience working with cars.

How to Use Bondo Body Filler

Like every auto body repair project, the first step is gathering your tools and ensuring you have everything you need. The tools you need to have on hand for repairing dents and scratches with filler include:

Body shapers
Bondo spreaders
Paintless dent repair kit
Stud gun
Dent removal tools
Body filler
Sander
80 grit abrasive sheet
Patch metal
Mesh
Mixing board

At a minimum you need to have the putty and spreader, but the rest is nice to have depending on the type of repair you're doing. If you have everything you need, then it's time to prepare your surface and get to work.

Step 1

First, you always want to prepare your surface. This means the vehicle needs to be completely clean - free of bugs, grime, dirt, wax, or any other substance that might keep the putty from sticking. This might mean a simple clean with soap and water, or a wipe down with residue or adhesive remover.

It also means that dents should be taken out, and rust and scratches should be sanded. Paint around the damaged area should also receive a good sanding, since the filler doesn't stick well to paint, wet, or humid surfaces.

*Tip: sometimes duct tape is used to cordon off the area of the damage and keep the rest of the vehicle from any residuals from the sander.

Step 2

Mix your filler (follow the instructions on your filler product) so that the hardener is completely introduced- producing a touch of pink in the mix. You can do this with your spreader and mixing board.

With a good mix in your hand, you can use your spreader to coat the damaged area completely, while avoiding over filling. Make sure this first layer is applied firmly so it adheres well to the surface.

You'll want to fill the damage, but also extend the putty outwards to the surrounding area so that when it dries, and you sand it out, you'll have a seamless transition.

(If you need to use shaping tools, this is the time to do it- while the putty is beginning to harden but isn't yet dry.)

Let dry completely (20-30 minutes).

Step 3

Now you'll need to smooth out the filler by sanding. Grab a sanding block with an 80-grit abrasive sheet and begin smoothing out the completely dry filler.

If you find that you've over-sanded and now have a low spot, you may need to reapply a layer of filler and wait until it dries before continuing.

You want to sand until the surface of the putty is smooth and level with the rest of the body. Make sure that you featheredge around the rim of the damage so that it blends into the surrounding area seamlessly. This may require a finer grit (180 grit), as you featheredge, which will help to remove any scratch lines.

Step 4

Once you've ensured that the surface is smooth and you've effectively leveled out the damage to be even with the rest of the body, you can apply primer, topcoat, and clearcoat.

Body Filler Tips

Follow these tips to ensure that your repair is what you desired, and that it lasts.

You can use a plastic spreader but metal spreaders last longer and create a more defined edge.
Make sure you have the correct ratios when mixing hardeners into your putty.
Don't use cardboard as a mixing palette, because it is absorbent and will mess with the chemistry of the mix.
Have a thinner on hand in case your putty is drying too fast.
Use heat lamps to help the surface dry if you are doing this repair in cold or wet weather.

Next steps

As long as you've followed the steps and tips, and used the right tools, your vehicle should look just as good as it did before the damage- if not better.

Need filler or other tools to get started on your surface repair? Auto Body Toolmart has body fillers, spreaders, dent repair tools, primers, and everything else you need to get the best results.