Painter versus plastics

The first material that comes to mind when we think about car bodies is metal. From the very beginning, the body was made of sheet steel, but over time its importance evolved due to the desire to reduce the weight of the entire car, which was necessary to achieve greater performance. Especially recently, when engines are heavily ‘tuned’, the weight of the car must be significantly reduced so that smaller engines do not stand out from customer habits.

Many years ago, the decision was made to use polymer and composite materials in car manufacturing. When visiting a car dealership, you may notice that the colour of the plastic parts varies from model to model. Usually, in the euphoria of buying a car, inexperienced buyers do not take the time to look closely at the colour of the entire car. However, a customer who receives a car after a repair will definitely take a closer look at the paintwork.

More and more plastic parts

The plastics used to make car parts are not only very flexible and durable, but also easy to produce and, above all, cheaper than metal parts. Plastic parts such as bumpers, mouldings, wings, wing mirrors, fuel filler caps and other body parts are now standard in car bodies. An increasing percentage of cars are made entirely of different plastic mixtures or composites. This requires restoration companies to have a very good knowledge of various repair technologies.

Colour difference

OEM paint shops in the automotive industry are usually separated according to the type of substrate to be painted and the different surface preparation. Metal parts are painted in a different paint shop than plastic parts.

There are several reasons for colour differences:

• Different painting methods, paint layer thickness, substrate colour;
• Different paint shades from different batches supplied by the paint manufacturer;
• Corrections on the production line and even repainting.

Of course, everyone tries to eliminate these problems, as in the case of the fuel filler flap, which is now made of metal again. When repairing painted car parts, we are in a better position if the plastic part can be welded or glued and repainted after a suitable preparation procedure. However, it sometimes happens that a plastic part that was not painted at the factory has to be replaced because we are unable to recreate the texture that was created in the injection mould using repair painting. Of course, there are textured paints available (CP 582 by Profix) that imitate the original effect in a similar way, but this may not be enough.

How to prepare for painting?

When preparing to paint a new item or repair a plastic part, we should determine the correct repair procedure and start by identifying the type of plastic the item is made of (the marking is on the unpainted inner side). These markings are now required for recycling purposes. There may also be parts that are not suitable for painting, such as PP, PE or POM. If there is no marking, we can try to paint the part. It also happens that the vehicle manufacturer has deliberately prepared a plastic component by covering it with a protective coating, and therefore we cannot paint it. Such protection of components is used to protect them from operational damage (weather conditions), as for example in the case of commercial vehicles, and the relevant information should be sought from the vehicle manufacturer.

Repair methods for new components

Firstly, it is worth mentioning a frequently asked question during training sessions, which concerns the painting of surfaces made of glass fibre reinforced polyester. It is generally accepted that we repair such an item according to the technology intended for metal parts (unless there are other manufacturer's recommendations). When repairing a factory-painted part, we focus on using plastic repair technology only at the point of damage.

When we start painting a new component, we must first remove the silicone-based chemicals that protect the injection moulds with the component and prevent the paint from being applied correctly and effectively. To do this, we use detergent and water in the first step, and then silicone remover after drying. After this initial cleaning, it is a good idea to heat the element (for 30 minutes at 60°C) or carefully use a heat gun (caution is advised here, as we may damage the element). After heating, wash the whole thing again (e.g. CP 015 by Profix) to remove any silicone residue that has come to the surface during heating. For degreasing plastic parts, it is worth using antistatic silicone removers (e.g. CP 012 by Profix), which, in addition to cleaning the surface, allow the removal of electrical charges. Electrifying components can effectively hinder metallic base coating and will also attract more dirt to the surface.

During the painting process of a new component, we can use the wet-on-wet repair method. We start by applying a primer to the plastic, the so-called adhesion promoter (e.g. CP 390 by Profix), and after it has evaporated, we proceed to the actual painting with 1K or 2K paint.

We can come across new elements on which the manufacturer has secured adhesion for the topcoat by applying its own primer. This makes the preparation procedure for painting faster - we can start by matting the surface, cleaning and degreasing it with the appropriate remover, and then proceed to the actual painting.

Some automotive companies sell spare body parts in the right colour for the car (e.g. HONDA), so we only need to replace the damaged part.

Painting methods for damaged parts

In the case of partially damaged painted parts, the repaired area is sanded and, after cleaning, plastic filler is applied (e.g. CP 341 by Profix). Then, after obtaining a suitable surface, the part can be cleaned and a plastic primer, the so-called adhesion promoter (e.g. CP 390 by Profix), can be applied. adhesion promoter (e.g. CP 390 by Profix). After it has evaporated, we apply the filling primer. If the plastic element is flexible, we add a plasticiser (e.g. CP 490 by Profix) to the filler primer. This is the same plasticiser that can be added to 2K paints to give the paint coatings the right flexibility and prevent cracking during use. It is best to dry the components at room temperature or in a spray booth at 50°C for 30 minutes. Please note that using higher temperatures in the booth or with an infrared heater can damage the component by deforming it.

After sanding the primer filler, we are ready to paint. All we need to do is clean the surface with plastic cleaner and we can start. If we have used a hardener with the 2K paint, we must take into account the need for a longer drying time of the repaired coating. The same preparation procedures for painting plastic parts apply to conventional topcoats as well as water-soluble base coats.

How to polish?

If the paint job has resulted in paintwork damage, we can try to remove it by polishing, according to the technology recommended by the manufacturer of the polishing materials. One of the most important limitations of mechanical polishing with a polishing machine is the rapid heating of plastics, which can damage the paint layer.

The final results?

Despite using the same products to paint plastic and metal surfaces, there may be slight differences in colour due to the fact that the metallic base paint will settle differently on a metal surface than on a plastic surface. The basis for successful painting and repairing of plastic is following the rules in the technical instructions of the paint manufacturers. Then the difference in the painted items on different surfaces will be invisible.