Why Isn’t My Urethane Hard Coat Drying?

Customers of urethane hard coats have called VFI asking why their urethane hard coat isn’t drying. Mix ratio, poor mixing, application thickness, and temperature can all cause urethane to not dry.

Urethane coatings can sometimes be hard to work with if you’re new to the material. Read the material technical data sheet (TDS) before you begin working to avoid errors.

Reasons Urethane Hard Coats Might Not Dry

Issue #1: A hard coat customer reached out about the coating not drying correctly over EPS foam. After talking through his issues, we discovered the problem. He was trying to use the hard coat more like paint. He sprayed in several thin passes at less than 20 mils per pass.

For a urethane hard coat to dry properly, a minimum amount must be applied to the surface. When applying to EPS foam, it must be sprayed thicker, or it won’t fill the foam in and will have a popcorn-like finish.

Issue #2: Another issue we’ve seen urethane hard coat users run into is using the wrong mix ratio. This is typically not an issue for those using spray rigs with proportioners. It is more likely for brushable hard coat users.

If you’re weighing the material by volume, you can mistakenly read the lines on your measuring cup wrong. Using too much of one material or not enough of another can result in your hard coat not drying.

Issue #3: You may notice the coating has some sticky spots after allowing it to dry for a few hours. The tackiness is unmixed material. Failure to thoroughly mix the material creates these spots.

Again, this is typically a problem for brush-applied coatings. However, off-ratio spots can happen in sprayed coatings if the proportioner becomes off-ratio at any point. When using cartridges, the mix may be off-ratio at the beginning or end of the tube as well.

Poor mixing can occur because users are worried about the working time and are in a rush. If you are using large amounts at once, your working time is reduced, which can also cause mixing errors.

Issue #4: Another factor that controls the drying time of a hard coat is temperature. This means temperature of the environment, application surface, and material. If you work in cold conditions, it can take your coating much longer to dry. In freezing conditions, the hard coat may never dry correctly.

Solutions

If your coating isn’t drying, you may have to start over or do large-scale patches. Follow the manufacturer’s instructions on the technical data sheet. Surface preparation, mixing, application, and drying instructions should all be included.

Work in a temperature-controlled (77°F) location. Warming up your material to at least 65°F can help it perform to standards. Avoid storing or applying in very cold or hot conditions for optimal results.

When weighing out your material, VFI recommends using the mix ratio by weight instead of volume. It’s easier to notice if you go above or below the amount of the material needed on an accurate scale.

Mix up your material in smaller amounts. This will help you avoid poor mixing by extending the working time of the material. Mix the material in one container until it is a uniform color. Scrape the sides and bottom of the container while mixing. Pour it into a new, clean container and mix again to ensure all parts are sufficiently mixed.

If you are using hard coat cartridges, make sure to spray on a disposable surface before spraying onto your piece to ensure no off-ratio material is used. When your cartridge is almost empty, spray onto the disposable surface again.

VFI recommends applying urethane spray hard coats at a minimum of 40 mil passes to create a uniform film that protects the piece. The minimum application rate for a brush-applied hard coat is a little higher at 60 mils. Depending on the application, you may want to increase the thickness for better impact resistance.

Increasing the temperature after application can help the material cure faster. Make sure the relative humidity is low so the coating doesn’t bubble.

Contact VFI if you have more questions about troubleshooting hard coat issues.