How to apply paint to plastic?

In the field of painting, impression and adhesion are essential because they determine the good hold and durability of the paint job carried out. The guarantee of good performance of a painting carried out on a support depends on the quality of the paint but also on the work carried out upstream: the preparation, the technique, the primer. Painting on plastic is a tricky business and a good level of adhesion is often difficult to achieve.

We may need to paint plastics in the bodywork sector (bumpers, Quad bikes, fairings, etc.) but also in the industrial sector (crates, glasses, tanks, etc.).

Here is a unique, complete and detailed tutorial on techniques for painting plastics in 4 steps:

Step 1 ► Examine the plastic
Step 2 ► Prepare the plastic
Step 3 ► Apply an adhesion primer
Step 4 ► Apply paint, varnish...

Here are the products you need to paint plastic:
♦ Degreaser
♦ Paint thinner
♦ Primer for plastic

Here is the equipment needed to paint on plastic:
♦ Abrasive sponge
♦ Gas torch/burner

Caution
Painting on plastic is more complex and problematic than you might imagine. The risks of detachment or reaction with the plastic are very high.

If you want to successfully paint plastic, you must also understand that the specific preparation of the plastic is at least as important as the application of the adhesion primer itself. Applying an adhesion primer without carrying out special preparation will not be sufficient to guarantee the good performance of a paint on plastic.

Step 1 ► Examine before painting a plastic

All measurements and techniques presented here are valid for painting raw, bare plastic. Of course, if the paint is intended to be applied to painted plastic, then we consider that the application is done on paint: this does not require any preparation or special primer, with the exception of a simple sanding.

Before painting plastic, you must examine the plastic surface and determine its nature.
Plastics always have an acronym on their inside: whether it is a fairing part, a water bottle, or even a toy, you can find an initial surrounded by a triangle, which is the identification marking that designates the type of plastic.

Knowing the type of plastic will allow us to know more precisely how to best prepare the plastic. This does not affect the use of plastic primer which, in all cases, must be used.

Step 2 ► Prepare the plastic for excellent adhesion

As a general and summary technique, preparing plastic consists of subjecting it to the most difficult treatment that it can withstand without being deformed. The differences in resistance between plastics are very significant, hence the interest in finding out about the type of plastic, and if in doubt, doing some tests.

► Sand
Here is the main technique for promoting adhesion on plastics. This consists of modifying the shiny and closed surface state of a plastic, by abrasion.

No abrasive paper should be used, but rather abrasive sponges (gray or green).
Correspondence
→ gray abrasive sponge: approximately P800
→ green abrasive sponge: approximately P320

When the sandpaper heats up and creates scratches or plastic filaments, the sponge scratches finely and perfectly. On the other hand, the abrasive sponge allows you to work very quickly and in the smallest corners, without difficulty.

The action of the abrasive sponge is sufficient to create grip on certain delicate or sensitive plastics such as PS and on the contrary, it is insufficient on other much tougher plastics such as PP or PE.

On delicate and fragile plastics, it is in most cases possible to use isopropyl alcohol to scrub and degrease without damaging the surface of the plastic.

On very resistant plastics, it is recommended to dip the abrasive sponge in thinner, in order to vigorously scrub the surface of the plastic. Generally, paint thinner is used.

After treatment and drying, the plastic, once shiny and colorful, now has a whitish and matte appearance: this is a good sign because it increases the chances of the paint adhering well to the plastic.

► Flaming: the technique of flaming plastics.

Flaming is a very effective technique (but should be used with caution) which allows you to create excellent adhesion on certain difficult plastics. We use this technique on bumpers, Quad fairings, PE tanks... Please note, this technique should not be used on certain plastics!

The technique is very simple: you just need to equip yourself with a small gas stove with a flat tip, which emits a blue or yellow flame. You should not use a heat gun, as it does not have the same effect as a flame.

With the flame, you have to make 1 or 2 rapid back and forth movements on the surface of the plastic.
We can observe a sort of sweating of the plastic which occurs very succinctly. The grip is not the result of “sweating the plastic solvents” but rather it is the result of the removal of electrical voltage from the surface. After applying the flame, we benefit from around twenty minutes during which adhesion to the plastic will be facilitated.

⚠ Do not overheat the plastic as it can quickly warp very quickly, especially if the plastic is not thick.

⚠ The flaming technique is reserved only for very resistant plastics such as polyethylenes (PE) and polypropylenes (PP).

⚠ Danger: work far from all flammable products or vapors!

Here is the list of the main plastics and the methods for preparing them.

Painting PET plastic:
This flexible, shiny plastic is very resistant to solvent. You can therefore scrub it with an abrasive sponge and thinner without moderation. However, it is fragile to heat and its shape can change instantly if you approach a flame. It is a relatively easy plastic to bond with plastic adhesion primer.

Painting HDPE LDPE PE PP plastics:
Here is a family of flexible and ultra-solvent resistant plastics. They must be rubbed roughly with thinner and an abrasive sponge and apply the flaming technique (The HD version means high density, the less solid LD version means low density, so take that into account!). These are the most common plastics in bodywork and unfortunately, the most difficult for paint to adhere to.

Painting PVC plastic:
This is a type of flexible and durable plastic. It is not at all resistant to solvents: if you apply a little thinner to its surface, it will create an indelible mark. This is a separate plastic for which there is a specific and ultra-effective liquid PVC primer.

Painting PS (Polystyrene) plastic:
Here is a plastic which can be, depending on the type, more or less fragile. It is therefore advisable to take care and carry out preliminary tests before applying a sanding solvent. We recommend dry sanding. This type of plastic cannot withstand flames and may crack or crack. It is a relatively easy plastic to hang.

Painting PC (Polycarbonate) plastic:
It is a rigid or flexible, glossy, solvent-resistant plastic. It is a very resistant plastic that is found, for example, in vehicle headlights or the bodies of radio-remote controlled vehicles.
It can be scrubbed with thinner and an abrasive sponge. This type of plastic does not withstand flames, cracks or cracks can appear very quickly. For this particular plastic, there is a unique and specialized primer, in a 100% water-based version, it is the primer for lexan or the Flexiplast varnish.

It is recommended to do the sanding first, then the flaming.

► Degrease
The final stage of preparation is degreasing. This step also has the advantage of eliminating static electricity, which is harmful to adhesion. Of course, this helps remove invisible contaminants.

When the plastic allows it, it is possible to clean with thinner.
When the plastic is fragile, we use a degreaser.

Step 3 ► Apply an adhesion primer for plastics

Once the part has been properly prepared, dried, dusted and degreased, then it is possible to apply the plastic primer. This product is available in variable form and may be called a “plastic adhesion promoter”. This is a fancy way of saying that the primer does not guarantee adhesion, it simply helps the paint to adhere to the plastic.

In the bodywork sector, there are adhesion primers for two-component plastics. We do not recommend them on truly soft plastics, as these primers are quite hard and thick, and as a result, they can crack and peel on soft plastics.

♣ In general, on all soft surfaces and plastic surfaces it is recommended to apply as little thickness as possible to avoid cracking and peeling.

We offer a primer for transparent plastic, which is particularly effective on a majority of plastics, particularly PP and PE. This primer is effective even without any preparation on many plastics (including PE).

The plastic primer is applied in 1 or 2 thin coats. It is a product that dries very quickly and is very fine. It is therefore recommended to apply the following paint directly (1 to 2 minutes later) above the freshly applied plastic primer.
You must apply a thin layer of paint to allow the plastic primer and the paint to blend together.

When it is not possible to paint quickly after applying the plastic adhesion promoter, it is then necessary to allow it to dry completely, then sand with a gray or green abrasive sponge.

Apply paint to plastic

Here's a good old rule: it is possible to paint any type of support as long as this support has been correctly prepared and/or primed.

This means that absolutely any paint can be painted once the plastic preparation work has been properly carried out.

If we take the example of a Quad
→ Without preparation, with a simple plastic primer, the life expectancy of the paint does not exceed 1 month, before it begins to peel off.
→ With scrupulous monitoring of the preparations detailed in the previous paragraph, the paint is completely protected from the risk of peeling and its life expectancy is as long as the paint allows.

Here are 2 tips on what to avoid!

⚠ Avoid the accumulation of layers and the formation of significant thickness: plastics are flexible surfaces and above all, they are often intended to manufacture parts exposed to shocks or projections. It is therefore advisable to paint the minimum thickness for the different layers of primer, paint and varnish.

⚠ Paints are naturally flexible. However, two-component polyurethane paints and varnishes are a little harder. Their rigidity increases with age, until it becomes rigid and brittle after a few years.
On flexible plastic parts, it is therefore strongly recommended to use an additive which serves to keep the coatings flexible without breaking.

The use of a flexibilizing additive is only necessary for 2-component polyurethane paints. One-component water-based or solvent-based matte base paints do not need this type of additive.

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