How to Fix Holes or Cracks in Urethane Hard Coat?

Accidents can happen when using urethane hard coats over EPS foam shapes, signs, and props. We’ve had customers call in about cracks, holes, and dents on their sprayed hard coat layer and are looking for a fix. While we’ve heard of many DIY options, such as Bondo, plaster, and other polyester resins, the best repair material is a brushable urethane hard coat.

What Is the Problem with Other Repair Materials?

The biggest reason you shouldn’t use other materials to repair a hard coat is because they won’t be as compatible. A brushable material like VFI-2519 75 D Brushable Hard Coat has similar capabilities as a sprayable material like VFI-6170 70 D Spray Hard Coat.

At their core, these coatings are both polyurethanes. When properly prepared, a new coat of urethane bonds extremely well to a previous coat. The brushable hard coat can create an effective seal on the old hard coat, so the repair appears seamless. The ability to sand and paint the hard coats is comparable as well, so it’s easy to blend the repair in with the rest of the coating.

Patching materials like Bondo or plaster won’t be able to behave exactly like a hard coat. Bondo is commonly used for autobody, boat, and home repairs on surfaces like wood, metal, concrete, and more. Plaster is typically used as a building material or to patch cracks and small holes in walls and ceilings. These materials may not adhere well to polyurethane coatings.

Another reason to use a brushable urethane hard coat is that it will be able to match the properties of the existing coating. At around the same hardnesses (70 D vs 75 D), they offer similar abrasion, impact, and wear resistance. While they are formulated to be rigid, they also have good flexibility to prevent cracking. They offer these benefits whether you apply them by spray or by brush.

While Bondo and plaster are very rigid after they have cured, they won’t allow for the same amount of movement. If the hard-coated foam surface moves when temperatures are elevated, the repair material will move with it. Eventually, the Bondo or plaster repair will crack or fail over time, which defeats the purpose of using it as a repair. Also, polyester can melt the foam underneath, which is more trouble than it is worth.

Because the hard coat is meant to be painted, Bondo will cause issues with the topcoat. When paint is applied to the polyester resin, it will off-gas and wreck your paint. If these repairs are large, your overall piece might not look good.

Ultimately, these materials are too dissimilar to the original polyurethane hard coat to stay bonded together and undamaged. A urethane-based material will offer better adhesion, durability, and flexibility for repairs.

Solution

So, while you can use Bondo or other materials to repair cracks and holes in your coating, it’s better to use a brush-applied hard coat repair like VFI-2519 or VFI-2626. VFI-2626 has fire retardant in its formula, so it’s the better choice for indoor projects.

How to Use the Repair

When repairing an existing hard coat, you will need to remove any broken-off pieces in or around the damaged area. You will also have to sand in and around the damaged area to ensure adhesion of the repair material to the previous coating. Once the damaged area is prepared, mix up your repair material by weight or volume.

Before using the repair material, it is best to let it sit in the cup and heat up a bit so it gets thicker. This will make it easier to fill the hole or crack. An alternative to this method is to add fumed silica to your mix. This addition will make your mix very thick, like a paste, so it fills holes or cracks even better.

Fill the crack or hole until it is level with the rest of the hard-coated surface. We recommend using a rubber scraper to smooth and remove excess material when it’s still in a tacky state. Allow the patch to cure before sanding or painting it for theming purposes.

Contact VFI if you need assistance using a brushable hard coat repair or if you have any other questions.