Contents
Paint peeling on plastic bumpers is usually caused not by the paint itself but by preparation steps that were skipped or done incompletely. With their low surface energy, mould release agent residues and flexibility, plastics call for a different process from metal. This guide takes you through the steps to follow, in order, for lasting paint adhesion on a new or repaired bumper.
First, Identify the Plastic
On the inside of the bumper there is a material code, usually written between “>” and “<” symbols. This code determines the preparation route to follow.
| Marking | Material | Painting note |
|---|---|---|
| >PP< | Polypropylene | Low surface energy; plastic primer is essential |
| >PP+EPDM< or >PP/EPDM< | Rubber-modified polypropylene | The most common type for bumpers; flexible, plastic primer is essential |
| >ABS< | Acrylonitrile butadiene styrene | Relatively easy to paint; use the primer specified by the system |
| >PC/PBT< | Blend of polycarbonate and polybutylene terephthalate | May be sensitive to some solvents; choose the cleaner and primer according to the Technical Data Sheet (TDS) |
Suffixes in the code such as T20 or TD20 indicate that the material is 20% talc-filled. Some plastics, such as PE (polyethylene), are not suitable for painting with standard paint systems. If the code cannot be read, the safest approach is to treat the part as PP-based and never, under any circumstances, skip the plastic primer step.
Tempering New Parts (Degassing)
New plastic parts may contain mould release agents left over from production, as well as volatile compounds trapped inside the material. After painting, the heat of the oven can bring these substances to the surface, causing bubbles, craters and loss of adhesion. The solution widely used in workshops is to temper the part by heating it before painting:
- Wash the part with warm soapy water, then rinse and dry it.
- Place the bumper on a suitable stand so that it cannot deform, and leave it in the oven, typically at around 60 °C for 30–60 min.
- Allow the part to cool to room temperature; any residues that have come to the surface will be removed in the next step.
An oily film or marks on the surface after tempering show that release agent has been driven out, which is exactly why the main cleaning is done after tempering.
On new parts supplied factory-primed, check that the primer is sound. If the primer softens or colours the cloth when you wipe an inconspicuous spot with a solvent-dampened cloth, follow the recommendations of the part manufacturer and the paint system before building a system on top of it.
Cleaning and Abrading
Cleaning
After tempering, all release agent, silicone and grease residues must be removed completely. Use a surface cleaner such as ZN-530 Surface Cleaning Thinner with the two-cloth method: wet the surface with one cloth and wipe it off with a second clean, dry cloth before the solvent evaporates. Before using the cleaner on a plastic part, confirm its compatibility with plastics in the TDS. Plastics readily build up static electricity through friction and attract dust; using an antistatic plastic cleaner for the final wipe and not rubbing the surface hard with a dry cloth reduces this problem.
Abrading
For adhesion, the surface needs to be keyed at a microscopic level, but deep scratches are undesirable:
- Abrade the whole area to be painted evenly with a grey abrasive pad and scuffing paste, or with P400 abrasive paper.
- Coarse abrasives cause fuzzing on PP-based plastics, and a fuzzed surface shows up as roughness under the paint.
- After abrading, wash and clean the surface again; do not leave paste residue in corners and recesses.
Previously Painted Bumpers
On a bumper that has been painted before, first check that the existing paint is sound; paint that is blistered, cracked or comes away easily must be removed completely. Abrade sound paint with P400 or a grey pad. As you feather the edges of scratches and scuffs with P320 and then P400, identify any spots where you have gone through to bare plastic; these areas will be sealed with plastic primer in the next step. Because the bumper surface may carry road grime, insect residue and wax, clean it twice: before and after abrading.
Plastic Primer: Thin, Single Coat and at the Right Time
The key step for lasting adhesion is an adhesion coat that bonds to the low-surface-energy plastic. ZN-240 1K Plastic Primer has been developed for exactly this purpose, for application to bare plastic surfaces.
- Apply the primer as a single thin, semi-transparent coat. The “thicker is better” rule does not apply to an adhesion coat; an unnecessarily thick film can create a weak intermediate layer.
- On repaired painted bumpers, apply the primer to all areas of bare plastic exposed by sanding.
- Observe the waiting time stated in the TDS before moving on to the following coats. If the time is too short, solvent entrapment can result; if the maximum time is exceeded, adhesion can be weak.
Primer, Basecoat and Clear Coat Layers
The system built on top of the plastic primer depends on the condition of the surface:
- Smooth new parts: If no filling is needed, you can move on to basecoat without sanding by using a primer applied wet-on-wet. ZN-250 Joker Multi-Purpose Primer, which works in both sanding and wet-on-wet systems, is a practical option for this workflow.
- Repaired surfaces or surfaces that need filling: Apply a 2K primer filler over the plastic primer and continue with a sanding system. In either case, check the TDS for the conditions and waiting times for applying the primer over plastic primer.
- Flexibility: On flexible parts such as bumpers, the primer and clear coat layers must be able to flex with the part. When using a flex additive, follow the paint system's TDS.
- Basecoat: Apply the basecoat with the number of coats and flash-off times specified in the TDS; shortening the flash-off before clear coat increases the risk of dulling and bubbles caused by solvent entrapment.
- Clear coat: After the basecoat, complete the job with a 2K clear coat such as ZN-320 2K Acrylic MS Clear Coat.
When oven-drying plastic parts, typically limit the object temperature to 60 °C and dry the bumper on a stand that keeps it in shape. As standard polyester fillers carry a risk of cracking on flexible parts, flexible products recommended for plastics should be used if filling is needed.
Common Problems and How to Prevent Them
| Problem | Possible cause | Prevention |
|---|---|---|
| Paint peeling off in sheets | Plastic primer skipped, release agent residue | Tempering, two-cloth cleaning, plastic primer |
| Cratering (fish-eyes) | Silicone or release agent contamination | Repeat the cleaning, use clean cloths |
| Bubbles and outgassing | New part not tempered | Tempering before painting |
| Dust and fibre pick-up | Static electricity | Antistatic cleaner, careful wiping |
| Surface fuzzing | Coarse abrasive | Abrading with a grey pad or P400 |
| Cracking when flexed | Insufficient flexibility | Flex additive according to the system TDS |
Tip: After abrading, look at the surface with the light at an angle. Every spot that is still glossy is somewhere the abrading has not reached and where the paint will lift first; check edges, corners and recesses in particular.
Summary
Lasting adhesion on a plastic bumper starts with identifying the material, tempering new parts, removing release agent completely, abrading the surface evenly with a fine abrasive and applying plastic primer as a single thin coat. A primer, basecoat and clear coat system that takes account of the need for flexibility then makes the result last. You can explore the product combinations we recommend for different substrates on our application systems page.