How To Use Body Filler: 9 Tips And Techniques

28 November 2022 Automotive

Car body filler is widely used for the quick repair of minor damage such as scratches, dents and treated rust perforations. Used properly, this two-component polyester resin can restore a vehicle to near-original perfection but if not, it can result in imperfections that lead to further work.

Here we give a few hints and tips on how to use car body filler for the best results. If you are a professional car repairer you may know most of this already, but read on – there might just be something you can take away and use.

Choosing The Right Filler

Car body filler is available in a range of formulations, each suited to a particular application. Let’s start with the products you are likely to use on more severe damage and work our way up to the finer finishing fillers.

Glass Fibre Body Filler

This filler contains strands of glass fibre that give it additional structural strength and a permanent, waterproof seal when bridging holes or cracks.

Deep Body Filler

Deep body filler is ideal for repairs that require thicker layering as it features excellent adhesion to common substrates and resistance to cracking.

Medium Body Filler

This filler provides a super-smooth finish that makes it ideal for feathering out the edges of a repair. It can be used as the base for finishing filler on moderate repairs or to refine the surface of a glass fibre or deep filler repair.

Glaze Body Filler

Glaze body filler is the product you will use to eliminate tiny surface imperfections and pinholing immediately before you start to prime and paint. It is easily abraded with light-grain sandpaper to provide a super-smooth finish.

Primer fillers can also be used on very slightly damaged areas such as scratches. Available in an aerosol, they are sprayable body fillers that require neither mixing with a separate hardener nor hand application but are sprayed on like paint and gently sanded back to a fine finish.

Primer Filler Aerosol

This is a high-build spray filler primer with excellent adhesion, fast-drying and easy-sanding properties. It can be used as the primer for water- and solvent-based paints.

Fillers are also made for more specialist applications. These include:

Plastic Filler

Plastic parts such as bumpers and trims tend to flex more readily than metal parts. This filler offers the flexibility and adhesion needed to prevent cracking and delaminating when flexing occurs.

Wheel Filler

Wheels are exposed to more stone chips, road traffic film and de-icing salt than most other areas of the car. This specialised filler is formulated to resist them more effectively.

How To Use Body Filler

Now that we’ve looked at the different types of body filler it’s time to move on to how to use it.

1.    Preparing The Surface

Start by making sure that the damage and its surrounding area is completely clean. A simple wash-down with soapy water and a good rinse may be enough, but if necessary use a more specialised product to remove stubborn traffic film, adhesive residue and so on.

TIP 1:         If you are filling a hole in a fuel tank, make sure the level of the fuel in the tank is well below the area to be repaired (or preferably empty) as petrol/diesel will dissolve the uncured filler.

Pull out dents, making sure that no parts protrude further than the finished level of the area.

Sand the area to remove any rust, chipped or loose paint and provide a key for the filler. Ensure all rust is removed, as this will create problems later on. if necessary, apply rust neutraliser to any difficult-to-treat areas.

It’s a good idea to lightly abrade the entire area that will be repainted while you are sanding the damaged areas, to provide a good key for your primer and paint..

TIP 2:         Apply quality masking tape to the edges of surrounding panels if sanding a complete panel. This will protect them against damage from the sanding.

Be aware of pinholing in welded repairs, as these can allow water to penetrate the repair and cause bubbling of the unprotected filler.

TIP 3:         Shine a light at the back of a welded repair and look for pinholes in the weld. If you find any, weld them to seal the repair and grind them flush.

2.    Mixing The Body Filler

Always mix body filler on a surface made from a non-porous material such as metal or plastic; as tempting as it might be to use a discarded piece of cardboard, this will absorb some of the chemicals from the filler and affect the filler’s performance.

TIP 4:         Never use your mixing tool to take filler from the container. Even if you think you have wiped it clean, the smallest residue of filler mixed with hardener can start a catalytic reaction in the container and can spoil the rest of the filler for future use.

It is important to mix the filler and hardener to the exact proportions stated on the packaging or container, as adding less or more hardener to slow down or accelerate the cure time is likely to result in problems – insufficient hardener can result in the filler not completely curing, while too much hardener can cause excessive exothermic heat during curing, more pinholes in the surface and lower cured strength.

TIP 5:         To extend the cure time of the body filler, work in a cooler environment. To accelerate the cure once the filler has been applied, use heat lamps.

3.    Applying The Body Filler

Apply the filler to the damaged area, making sure that it is pressed firmly and fully into the surface to prevent air pockets that will reduce adhesion and strength.

Frequently wipe the edge of the applicator to remove any filler that has started to dry, as this will leave tramlines in the surface of the filler that will require further filling.

Extend the filler beyond the area of the repair so that it can be feathered out during sanding, but avoid excessive overfilling as this will increase the amount of sanding required later.

TIP 6:         If you are filling a large area, use a steel rule as an applicator. This will bend to follow the contour of the car panel but remain rigid enough to keep a uniform curve across unsupported areas.

If any shaping of the filler is required before it is fully cured, wait for the filler to harden slightly. If it starts to crack while you are shaping it, wait until it is fully hardened.

4.    Sanding The Body Filler

When the filler has fully cured (generally after about 20-30 minutes) use a sanding block with a coarse grit paper to gently remove any protruding filler, sanding it back to the finished level with increasingly fine grits and feathering the edge of the filler to blend it in with the surrounding area. Don’t worry about low spots in the filler, as these can be fixed with a second application.

TIP 7:         Use your steel rule again to check for any high or low spots in the filled surface. Shine a light on one side of the rule and look for any visible lines of light showing between the rule and the filled surface on the other side.

Mark on the surface where the high spots need sanding (you can sand away the marks and then check the levels) and low spots need filling.

TIP 8:         Filler doesn’t stick well to pencil graphite, so use a spirit marker or another non-graphite, wax- and grease-free marker.

If needed, apply a second coat of filler to the low spots and repeat the sanding and checking process until you are satisfied with the result.

Now check the surface for any microscopic blemishes such as sanding lines and pinholes.

TIP 9:         Use a Dry Guide Coat to highlight small blemishes. This is a black powder that when applied with a sponge to the area will settle into any small scratches and holes.

Depending on the severity of the blemishes, you may choose to sand them out with a finer grade of sandpaper, or use a fine filler or even a primer filler.

Wet sanding with wet and dry paper gives a faster cut and a finer finish than dry sanding, but as filler absorbs water, leave it to dry out fully (at least 24 hours) before applying primer or paint if you have wet-sanded. Wet sanding can also create fine surface rust on bare metal, which must be removed and neutralised before applying further filler, primer or paint.

Is Your Body Filler Ready To Prime And Paint?

It can feel time-consuming and laborious to repeatedly fill and sand a repair, but the surface needs to be right before you start priming and painting; it’s surprising how even the slightest scratch or unevenness can still show after the final gloss coat is applied. We recommend using Dry Guide Coat again before and after applying primer to check for those last tiny blemishes that might have escaped your inspection.

Once you are ready to move on to the next stage of your repair, check out the ProXL range of professional primers.