9 essential tips to repair a plastic effectively

Plastic repair in the bodyshop has established itself as a repair process that every bodyshop professional should be familiar with and know how to perform. However, in addition to knowing how to repair plastic, it is essential to be aware of the following tips. Do you want to know what they are? We’ll tell you about them below.

What are the characteristics of a plastic?

Before explaining these tips in detail, it is important to know the general and specific features that define a plastic, since they determine the repair process to be followed.

The most relevant general characteristics of a plastic are as follows:

They are classified into three distinct groups: thermoplastics (for parts such as bumpers that must show some flexibility), thermosets (for parts that must be flexible but rigid and hard such as front panels or impact absorbing cross member) and elastomers (used in the manufacture of rubber items such as beading, sealing gaskets, etc.).
They weigh little.
They are resistant to corrosion, although they also oxidize and lose their properties when they are exposed for long periods of time to certain climatic conditions (sun, dew, salinity, etc.).
Most are easily recyclable, although the process is complicated when reinforcements or fillers are present.
In general, they show low adhesiveness.
They are thermal and electrical insulators.
They have the capacity to absorb part of the energy generated in collisions of a certain intensity.
Their level of flexibility varies depending on the type of plastic and the group to which it belongs.
They tend to become charged with static electricity.
They are sensitive to certain solvents and chemicals.
It is possible to repair them.

In relation to the specific features of each of the above-mentioned groups of plastics, these are the most important aspects:

Thermoplastics	Thermosets	Elastomers
They are flexible or semi-rigid. They soften and flow when exposed to heat and certain solvents. They can be mixed and reinforced. They are generally repaired using heat (weldable), adhesives or industrial acetone (ABS only). Some examples of these plastics are: PP, PA, PE, PVC, PC, ABS, etc. These plastics can be mixed with each other, such as PP+PE, or reinforced.	They are rigid. They only show sensitivity to heat when the decomposition temperature is reached. They are usually reinforced. They are usually repaired with resins and glass or carbon fibers. This group includes resin-based plastics such as EP and also includes thermoplastics reinforced with fillers such as talc (e.g. PP-T10) or glass fiber (e.g. PP-GFK30).	They are elastic. They soften and flow when exposed to heat and certain solvents. They can be mixed and reinforced. They are not usually repaired in the bodyshop. They are the least common. Some examples of these plastics are polyurethane and rubber.

9 Essential tips to repair a plastic effectively

1. 1. Make a correct assessment of the damage and type of plastic

The most frequent types of damage to plastics, beyond purely aesthetic damage, are the loss of material, cracks and broken anchors. In the event of any of these types of damage, an exhaustive assessment must be carried out in order to determine the feasibility of the repair compared to replacement in terms of the cost of such action and the need for it not to affect safety.

1. Effectively identify the type of plastic

Before starting the repair, it is essential to know the type of plastic you are working with in order to be able to place it in one of the above-mentioned groups. The bodyshop professional can then choose the most suitable repair method, tools and products.

Choose the right products for the repair

Plastic is a material that has very specific properties, so the products chosen for its repair should be specific for plastics whenever possible. Generally speaking, these products must offer better adhesion and a specific flexibility to match the mechanical properties of the plastic, as well as being less aggressive.

Thoroughly clean the surface

Any plastic used in a vehicle that undergoes a repair may be contaminated with some type of grease, oil or chemical such as silicones. The presence of contaminants on the surface of the plastic reduces the adhesiveness of the various products used and can cause rejection reactions that can spoil the paint finish. It is therefore necessary to clean the work area thoroughly before starting the repair and applying any products. In addition, it is recommended that the cleaner used be compatible with this material, such as Roberlo’s Blucleaner degreaser, in order to eliminate the risk of chemical attack and reduce static charge.

1. Prepare the surface

When repairing a plastic, the damaged area must be left with a surface condition that promotes the integration of the products and filler materials. This increases the mechanical strength of the work and integrates the repair into the rest of the part to make it invisible from the outer side. The preparation operations to be carried out (not necessary for ABS plastics) are as follows:

When the damage is a crack, the end of the crack must be drilled to prevent it from cracking further. The drill hole should be about 3 mm.
The joint edges (in cracks or jointing of split materials) or the contour of the damage (when there is a lack of surrounding material) should be beveled to increase the contact surface of the filler material, which allows the strength of the repair to be increased. The beveled angle made must be 45º when it is on one side only, and 30º when it is on both sides.

Finally, it is also essential to remove the paint from the bordering area and sand the entire area to achieve better adhesion.

1. Activate the surface

Any thermoplastic plastic, whether pure, alloyed or reinforced, has a low adhesiveness, which has to be improved with products that increase the grip. To achieve this objective, products with specific adhesion for plastics or adhesion promoters or primers, such as ROBERLO’s Plastic Primer, are sold.

1. Fill in the damage

After the substrate has been prepared, the damage is filled to form a single part once again. The filling is done on one or both sides, depending on the beveling that has been performed, and is done in one of the following ways:

With a two-component plastic specific urethane adhesive, such as ROBERLO’s Bumper Plast. This system is universal and valid for any type of plastic, especially thermoplastics.
By welding using plastic material of the same nature as the base plastic. This repair method is only valid for thermoplastic plastics as long as the polymer has good weldability. To determine this, a welding test must be carried out using this welding technique.
In ABS plastics, a paste of this material is obtained by mixing its chips with industrial or pure acetone.

1. Reinforce the inner side

The reinforcement of the inner side of the plastic increases the strength of the repair to make it more durable. The following reinforcements are used in each case

When two-component adhesive is used, the reinforcement is normally made with self-adhesive fiberglass mesh or metal staples
When the repair has been carried out by welding, the most commonly used reinforcement is metal staples or mesh.

Apply putties for plastic and sand them using the appropriate technique.

After applying the above-mentioned filler and integrating it into the rest of the part, a specific putty for plastics, such as ROBERLO’s Multiplast, is applied to provide better adhesion and greater mechanical strength. Finally, it is essential to sand it following the technique shown in our article “Fundamental aspects of the sanding process”. As a result, the patch is leveled and integrated into the rest of the part, leaving it ready for painting processes.

It is possible to repair a plastic and this can be done relatively easily. But remember that it is necessary to know the properties of these materials and apply the tips given throughout the article to achieve the best possible levels of finish, strength and durability.