How to use automotive body fillers

21 April 2022 Automotive

Your step-by-step guide to a professional finish 

Body fillers are widely used by the car body repair trade as a lower-cost and faster alternative to replacing entire panels where damage is repairable or replacements aren’t readily available. This also helps to eliminate environmental issues resulting from scrapping old panels. 

Mixed from a two-part polyester, they harden quickly, sand easily and give a perfect finish.  At least, they can do if they are used correctly. Let’s talk about how you can get a perfect finish on your repair or restoration project. 


1. Use PPE 

Polyester fillers contain substances that can cause complications if they come into contact with skin or are inhaled either in their uncured state or while being sanded. 

Always use appropriate personal protective equipment (PPE) – disposable gloves, protective overalls, a properly ventilated mask and eye protection are all recommended. 

Car body repair - filling and grinding pre-paint

2. Prepare the surface 

The panel may not be immediately ready for filling. Loose paint and corroded metal should be removed and rust should be treated. If the panel is dented, a hammer and dolly other dent-pulling tools should be used to restore it as closely as possible to its original profile. This doesn’t need to be too precise; just make sure that no parts protrude above the finished surface level. 

The next stage is to sand the surface; this provides a good, clean key for the filler. If you are using an DA sander, we recommend using heavy-grade (80-120 grit) sandpaper. If you are sanding by hand, drop this back to a lighter grade (180-220 grit) as this is less likely to leave deep sanding lines that may be visible later in the process. Red Scotch pads, available from all motor factors, are another alternative for preparing the keyed surface. 

When the repair has been sanded down and given a good key, use a degreaser to thoroughly clean all surfaces before filling. 


3. Choose your fillers 

If the panel is cracked or rusted through, the repair may need a glass-strand filler such as ProXL UltraFill Glass Fibre Body Filler, as the glass strands mixed into the polyester add strength and crack-resistance. 

For deep-filling a repair, you will need to use a high-build filler such as ProXL UltraFill Deep. This is a smooth textured, high-performance polyester filler that adheres well, bulks easily and Is easy both to work with and to sand. 

A lightweight, multipurpose polyester filler such as ProXL Ultrafill Medium is a useful second-coat filler for repairs that are either shallower or need to be feathered out, again offering excellent adhesion, workability and sanding. 

While the above products are designed to give primer-ready results, if you want to go the extra mile and create a perfect finish, use ProXL UltraFill Glaze for your final skim coat. This product is also useful for very light filling work such as surface scratches. 


4. Mix the filler correctly 

Before you start mixing the filler, make sure your area and tools are suitable. Clear the area of anything that might release contamination such as dust or other particles and grease or oil. 

Make sure the tools you are using to mix the filler and the surface are clean; bits of hardened filler from previous jobs can become dislodged into the new filler and spoil the surface. 

Always use a non-porous surface to mix the filler on. A piece of cardboard might be handy, but it can absorb some of the product and affect the cure. It can also entrap air into the mix, resulting in pin-holing of the surface. 

Don’t mix too much filler in one batch, even if you need a lot for the repair. By mixing smaller batches that can be used while the filler is at its most workable stage you won’t end up throwing away filler that has started to cure too soon or is more than the repair required. 

Follow the instructions for mixing proportions as closely as possible, as you are likely to get problems if the filler and hardener are mixed in the wrong proportions – adding more hardener ‘for luck’ can cause incorrect curing, a brittle fill that is prone to cracking, peroxide bleed-through on the finished paint job and increased chances of bubbling. 

It’s also important that the filler and hardener are thoroughly mixed, as pockets of filler without hardener will never cure properly. Keep mixing until there are no visible streaks from the coloured hardener. 

Smoothing car body filler prior to sanding

5. Apply the filler 

Avoid the temptation to overfill the repair, as this will leave you with a lot of sanding later on. Apply the filler in layers, using full, even strokes and smoothing and shaping it with the applicator. Let the filler dry between coats and sand it lightly before applying the next coat. Its good practice to use a guide coat, like the ProXL Dry Guide Coat, to highlight the high and low areas on the filler surface that may have otherwise been missed. 


6. Let it cure 

If you are carrying out the repair in cold or wet weather, it’s a good idea to use heat lamps to help the filler cure. Once the filler has fully cured, try to get the primer coat on as quickly as possible; unprimed filler can absorb moisture from the air, which can cause problems during the painting process.