Why Is My Urethane Hard Coat Bubbling?

Customers of polyurethane coatings have called VFI about bubbles forming while mixing their material or when the material is curing. Bubbles can form for a handful of reasons, but when it comes to urethane, it’s usually due to moisture.

Urethane is a two-component material, and its A side (Iso) is very sensitive to moisture or water. Wherever there’s moisture in the environment, it will react with the coating and form carbon dioxide gas, which causes foaming or bubbling.

Reasons Bubbles Form in Urethane Coatings

Bubbles can occur regardless of your application method (high-pressure spray, Qwik Spray, or brush-applied). Some methods with a longer application time will be more sensitive than those with a shorter gel time. In most cases, bubbles are usually small and not always visible. However, it is best to avoid the following:

1. Humidity

Sprayable coatings don’t have much time to react with moisture in the air, but if the humidity is relatively high, you might experience bubbling. This is also true if you apply the coating while temperatures are fluctuating.

Brushable coatings like some urethane hard coats are a slowed-down version of a typical sprayable urethane. Their longer pot life and cure time allow for more moisture absorption to occur during the mixing, application, and curing process. When you start mixing the components in a humid environment, bubbles will form and continue to form as the material is applied and left to cure.

Spray coatings formulated to be sprayed through cartridge guns can also experience moisture issues. The material reacts slower and comes out at a lower pressure, so it can’t avoid a moisture reaction.

Products like VFI-6171 70 D Qwik Spray Hard Coat and VFI-2538 QS 70 D EPS Form Hard Coat are known to create a finish that is lighter in color and with more of a surface profile. That texture is a result of the material foaming as it interacts with moisture, even at a lower humidity.

2. Wet or Porous Surface

If you put a non-breathable coating over a wet or porous surface, any moisture underneath will be trapped. Hot temperatures can cause the surface to expand and increase pressure, and the coating will form bubbles or blisters.

Allow the surface to dry before application. If you’re working with a porous surface, you may need to seal it with a primer. For example, wood surfaces should be dry and contain less than 11% moisture; otherwise, they should be primed or mist-coated.

However, the coating may bubble if the foam isn’t allowed to age for 30 days after it has been manufactured. By coating the foam too early, you might seal gases within it, which could cause it to bubble as it cures.

3. Wet Application Equipment

For spray coatings, be cautious about water getting trapped in the spray hoses. A moisture reaction could occur if water gets in from humid or cold weather. The A side will react with the water and cause it to cure in the line. The B side will take in the water causing a foam interaction when sprayed. When the mixed material comes out of the gun, you’ll notice bubbling defects in the finish.

If the formula allows it, some applicators use chip or foam paint brushes to apply hard coats. Make sure that the application equipment doesn’t absorb moisture from the environment.

Solutions

Work in temperature-controlled conditions with low relative humidity. We recommend applying the material around 72°F. Cold weather can slow the coating’s cure, which could allow more bubbles to appear in the finish.

Avoid using hard coatings over wet or damp substrates. Make sure your surface is dry and prepared. Porous surfaces like wood might need to be primed. If you apply it over wood, it should contain less than 11% moisture. If you’re working with foam, allow it to sit for 30 days for any gas to escape.

If you’re spraying, ensure your lines are clear of moisture contamination. They should be cleaned after each use with a compatible pump flush or lube like VFI-8005 or VFI-8011. For brush applications, it’s best to use non-shedding chip brushes to avoid moisture absorption that could occur in foam ones.

If the coating is hard enough, bubbles can be sanded to achieve a smooth, uniform finish. Work with 120-grit or finer sandpaper to scuff-sand the bubbles. You’ll want to apply another coat to cover those defects after sanding.

Contact VFI if you need other technical assistance with urethane coatings.