Why Can’t I Paint My Urethane Plastic Part?

Because urethane plastics tend to have high surface energy, most can be primed and painted, providing part makers with a range of color options to improve aesthetics, create a production quality finish, or blend with other parts. If you’re having issues painting your industrial, rotocast, or prototype part, there may be something on the surface preventing the paint from adhering properly. Alternatively, the paint or primer you’re using may be incompatible with the plastic.

Most VFI urethane plastic products are neutral or white after cure and are made to be fully paintable. Make sure to read the technical data sheet for the specific product you are using to ensure you are creating a good surface that will accept paint.

Reasons Urethane Won’t Accept Paint

1. Poor Surface Preparation

The biggest reason paint fails to adhere to a urethane plastic part is due to poor preparation of the surface. Issues will typically arise most in indentations, creases, or other hard-to-reach areas. Poor surface preparation can mean that you have failed to do the following:

     a. Clean the Part

As with any coating application, it’s essential for the surface to be clean for proper adhesion. After demolding and allowing the new parts to sit, they may attract dirt, debris, or oils from being touched. Leaving these contaminants can disrupt the adhesion of the paint you’re trying to apply.

Another contaminant is mold release. The majority of urethane parts are cast into metal or silicone tooling. While release is not required for silicone molds, it may still be used to ensure longevity of the tooling due to the wear and tear that urethane can cause. For metal or urethane prototyping molds, release is necessary; otherwise, the materials will bond together. If any release residue is still on the plastic part when you try to paint it, you’ll have issues getting it to stick. The paint may even bead up on the surface. And some release agents are hard to remove, so you need to be cautious.

     b. Prime the Part

Urethane surfaces typically turn out glossy, smooth, and non-porous, so it’s hard for paint to grip onto. Not preparing the surface with primer can prevent the part from accepting the paint. Even if a primer is not always necessary, sometimes it’s recommended to ensure that a bonding layer is created for the successful painting of parts.

     c. Sand the Part

The surface of the urethane after cure will not be acceptable for painting if it hasn’t been primed. Alternatively, if the surface is not abraded in some way, the paint might not be able to grip onto the surface. The shinier and smoother it is, the harder it will be for the paint to adhere long-term to the part.

2. Poor Mixing / Mix Ratio

If your urethane part feels oily or sticky after it has cured, this can be a problem when applying paint. The paint might not dry or may come off the part easily. It’s likely that either the material wasn’t mixed thoroughly enough, which resulted in uncured material throughout the part, OR the wrong mix ratio was measured, which resulted in too much of one material (typically the B side, Poly) being used. Mistakes during this part of the casting process are usually due to rushing when using plastics with limited pot lives.

3. Incompatible Paint

Not all paints are compatible with urethane plastic. While urethane adheres to almost anything in its liquid state, it doesn’t have the same properties once cured. Based on the properties of the paint and the urethane, some generic paints might crack, peel, or scratch off the part easily. If you use a highly rigid or DTM (direct to metal) paint, it might not have enough flexibility to be compatible with the urethane part.

Solutions

When mixing the material, make sure you use the proper ratio by weight or volume to ensure you end up with a fully cured part. Also, make sure you are mixing thoroughly to prevent streaks of unmixed material that may affect the surface of your part. If you need a longer working time, find a material that accommodates this, so you can manufacture quality parts. Allow the part to cure for several days before painting.

Wash your part to get rid of any release residue, dust, grease, and other contaminants. Typically, a mild detergent and water are recommended. Some part makers have seen success by wiping it down with isopropyl alcohol. Also, some release agents won’t make it as difficult to paint your urethane part, so consider finding one that won’t interfere with painting.

If you want to avoid using release altogether so you don’t have to worry about another post-processing step, use platinum silicone tooling over metal or urethane.

Apply a primer specifically made for plastics, like an automotive primer. In some cases, rather than priming the casting after demolding, some part manufacturers put a light coat of primer on their tooling surface after a release agent is applied. The primer will bond to the urethane plastic and should come out of the mold ready to paint. Allow the primer to dry for the time recommended by the manufacturer to get the best bond.

If you don’t want to use a primer and don’t need a smooth finish, sanding the surface will be your next best option. Lightly scuffing the surface with fine-grit sandpaper or sandblasting will create a rough surface for the paint to adhere.

You want to find a paint that will provide high adhesion to the part. Several acrylic and autobody paints have been proven to work well for these parts. Individual testing for paint compatibility may be necessary. When applying the paint, apply multiple thin coats as specified by the manufacturer to avoid excessive drying, warping, or poor adhesion between layers.

If you don’t want to worry about painting your part, find a pigmentable plastic that you can tint to your desired color. If necessary, ensure the plastic has UV stability to prevent color shifting from occurring.

Paintable VFI Plastics

Contact VFI if you need help finding the best paintable urethane plastic for your project.

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