The car filler or bodywork filler is one of the most commonly used items in the process of preparing and painting bases in any bodyshop.

It is a very thick paint that is easy to handle, and it comes in paste or cream format. It is used to repair and level out irregularities or dents produced on the surface of the bodywork, preparing it for the subsequent process of applying primer and finish paints. 

Although most body fillers are very versatile and can be used on different supports, some types specially designed to be applied on a particular material, such as plastic fillers.

The right choice and application of the filler is a critical aspect of doing quality work, allowing the professional to increase profitability with regard to the time and material consumed on every repair.

Characteristics of bodywork filler

At present, the most commonly used type of car filler in paint workshops and bodyshops is polyester filler, which is a type of filler that is composed of polyester resins.

Polyester filler is a product that is made up of 2 components or 2C, since it needs a specific benzoyl peroxide catalyst (in a ratio of 2 or 3% of the weight, according to the technical data sheet) to be activated. When the filler is mixed with the catalyst, it produces an exothermic chemical reaction (which generates heat) which causes the filler to dry and harden.

Componentes de la masilla de poliester y catalizador

The use of bodywork filler allows you to even out a metal or plastic surface, filling and levelling the area to be repaired. It plays an essential role as support and adherence element for base paints and its correct application will also influence the final behaviour of the finishing paints. An essential characteristic of a quality filler is good sanding and levelling behaviour.

It should be noted that fillers do not generally provide metal corrosion protection, so if your intention is to provide protection to bare sheet metal, you should first apply an anti-corrosion primer

In addition, the polyester filler cannot function as an anchorage base for final finishing paintsbecause, after sanding, you must apply a priming coat which will act as an insulating and anchoring coat.

Among the main characteristics that a good bodywork filler must provide, the following stand out:

Low porosity and high elasticity

A quality filler should not generate many pores when applied, minimising irregularities on the surface. 

In addition, especially on flexible supports such as plastics, the filler must provide elasticity to absorb possible impacts and vibrations, thus preventing cracking.

Easy application and sanding of the filler

The filler must be easy to apply and spread, resulting in a regular and homogeneous surface. In addition, it should make the sanding process easier, without having to apply excessive pressure or give several coats. This will result in less repair time.

Facilidad de aplicación y lijado

Good resistance to compressive and tensile stress

Sometimes, if the micronage of the filler coat applied is excessive, when compressive or tensile stress is applied, the filler coat and the topcoats of paint may tend to crack or peel. The concept of “level of fatigue” of the filler, i.e. the number of times it can withstand the same mechanical stress without deteriorating, is also important.

Correct Adherence

The polyester filler must have good adherence to the support. It is essential to use the appropriate filler for each surface, since each type has its own characteristics to optimise anchorage and adherence to the different materials.

Types of car fillers

In the past, bodywork fillers had thicker and coarser finishes, forcing the professional to slow down the filler application and sanding processes.

Over time, vehicle manufacturers have evolved bodywork designs, resulting in more complex geometric shapes and design lines, using new easy to shape and lighter weight materials such as plastic or aluminium. 

The appearance of these new materials in bodywork has also forced manufacturers to develop new types of specially adapted filler, without detracting any loss of adherence, hardness, drying and sanding.

Today, the wide variety of types of filler has allowed to improve productivity and profitability in repairs, offering in turn, a better finish.

This technological evolution and the appearance of different types and compositions of fillers, also forces the bodywork professional to know each one of the variants that can better adapt to each type of repair:

Universal polyester filler

It is the most commonly used type of filler in bodyshops. It has great hardness and resistance, and it also provides good adherence to steel plate, galvanising and electrozinc coating. It also stands out for its versatile use, both in car refinishing and in industrial vehicles. A recommended product from this category of fillers would be URKI-FAST. We could also highlight the URKI-2000 filler in this section. It is easy to sand, apply, thin and flexible.

Lightweight or low-density fillers

These are low-density fillers, with hollow charges, which makes them lighter. It is a type of  easy-to-sand and apply polyester filler, which is versatile and has a good level of adherence on different supports such as galvanised aluminium, plastics, steel, etc. Examples of this type of filler: URKI-LIGHT or URKI-LOW.

Filler for plastics

These are recommended for levelling deformations in plastic parts. It is a type of filler that has a high shock absorption, and high elasticity and flexibility. It can provide good adherence to the different types of plastics in a car, such as bumpers or exterior mouldings. They tend to have a softer and less porous texture. A filler in this category would be MASI-FLEX.

Aluminium content filler

This filler contains  aluminium metal pigments. This metallic component allows to increase the filling performance and hardness in comparison to other fillers. Aluminium particle-content fillers offer great adherence on different types of surfaces such as steel, aluminium or zinc coated substrates. A recommended product would be BESA-SILVER.

Fibreglass reinforced filler

It is a type of very compact polyester filler which has been added fibreglass, making it very hard and with filling power, in addition to making it very adherent to different supports and materials. Examples of fibreglass reinforced filler: URKI-FIB and BESA-FIB.

High filling power polyester filler

These are mainly used when you need to apply a thick coat of filler, mainly in major repairs where the part could not be levelled correctly by manual handling of the sheet metal.

They are used for small repairs where small holes have been made in the bodywork. You can find different load sizes to better suit each repair. For example: URKI-FAST

Gun spray-on filler

These are used for large surface filling. Their application is non-porous and uniform.

These fillers allow less thickness, and are applied with an airbrush instead of a spatula, in order to optimise filler process times: BESA-ROC.

Filler for touch-ups

Single-component polyester filler based on acrylic resin and drying only by air evaporation. They are used to cover small pores or scratches; they have low mechanical resistance and must be applied in thin coats. Its filling capacity is more limited. A recommended product would be URKI-KOMBI.

Tips for handling polyester fillers

As we always point out, it is advisable to take extreme precautions when working with chemical products such as polyester filler:

  • Use of appropriate PPE: During handling, you should wear gloves, a sweatshirt, goggles and a suitable respiratory protector.
  • Cleaning of equipment and tools: In order to prevent deterioration of the tools and equipment you use daily; you should pay special attention to their cleanliness. Application tools should not have any traces of the filler or catalyst used in previous repairs, in order to make a smooth and regular application on the surface. It is also advisable to carry out the mixing in a clean place designed for this purpose, to avoid contact with other products that may cause a possible chemical reaction.
  • Controlled environmental conditions: You must ensure proper ventilation to prevent inhaling toxic fumes and avoiding exposing the filler to extreme temperatures.