How to Use a Brushable Hard Coat + Testing for Recoat Window

Knowing how to use a brushable hard coat can save you money on labor costs and material waste. Before you begin working with new materials, always read the labels on the supplied containers. If it is difficult for you to read the labels, VFI offers technical data sheets with thorough instructions on every product page under resources.

If you’ve never worked with a brushable hard coat, you can also check out our detailed how-to video here.

Through this article, we will provide various tips for using one of these products. An important aspect of using a brushable hard coat is adhering to the recoat window. So, we also wanted to show you how we test for that.

Preparing Your Foam for Hard Coating

Typically, users apply these coatings to EPS foam. We recommend 2 PCF foam for the best cost-to-quality ratio. Allow the foam to age for 30 days minimum before coating it.

After carving the foam, it must be free of particles that might disrupt adhesion. Clean off the piece with a vacuum or compressed air. Also make sure the piece isn’t damp as this could also affect adhesion.

PPE for Applying Brushable Hard Coats

VFI recommends using personal protective equipment, including gloves, long sleeves, and safety glasses, when working with these materials. Please see the material SDS for more information.

Even though the coating is not sprayed, a respirator is required when sanding. Inhaling or ingesting these materials as particles can be toxic.

If you have a spray booth, it’s a good idea to work inside it while sanding. The fans will help keep the sanded hard coat particles inside the booth and away from your facilities.

Preparing Your Workspace

First off, these materials are sensitive to moisture. It is highly recommended that you work in a temperature-controlled environment around 72°F with low humidity.

If you don’t, bubbles will accumulate during mixing and application. These bubbles will then cure on and in the finished surface. The entire piece will require sanding and a new coat, depending on your desired finish.

Measuring the Materials

These are two component materials with a mix ratio of 1A:3B by volume. Since the B side is thicker and more of it is needed, we recommend measuring that material first. It’s much easier to pour the A side into the B side when ready.

We also recommend measuring how much of each material you need by weight rather than by volume. To do this, you will need an accurate gram scale. It helps you avoid making errors that could occur when measuring by volume.

Another tip when using these coating materials is to never use a full kit of material. Doing so will shorten the working time, which includes the time it takes to mix and apply it.

• VFI-2519 75 D Brushable Hard Coat has a mix ratio of 1A:3.44B by weight. To make calculations easier, we recommend using 100 grams of A side to 344 grams of B side. For an even smaller amount, use 50 grams of A side to 170 grams of B side.
• VFI-2626 65 D Brushable Hard Coat has a mix ratio of 1A:3.55B by weight. To make calculations easier, we recommend using 100 grams of A side with 355 grams of B side. For an even smaller amount, use 50 grams of A side to 175 grams of B side.

Note: when you’ve set aside the material you will be using, nitrogen purge both containers before closing them to extend the life of the unused material. You can find nitrogen purge kits on Amazon if you don’t have one readily available.

Mixing the Components

The second the materials touch, a reaction occurs, and the pot life begins. You’ll want to mix them using the double-bucket mix method. We recommend using a 1-gallon paint stir stick. You can find them at any home improvement store. Make sure to scrape the sides and bottom of the mixing container. Once you are done mixing in one container, pour the material into a new, clean mixing container and mix again.

It’s somewhat easy to tell when you’re done mixing because of how different the materials look. The Iso (A side) is very dark, and the Poly (B side) is very light. When the combined material is a uniform, neutral color, you’ve probably mixed it thoroughly and can begin applying it. You don’t want streaks of either material in the mix; otherwise, areas in the coating will be tacky and may never cure.

Applying the Brushable Hard Coat

When applying the material, use a non-shedding chip or foam brush. You can use a roller if you desire, but it must be a sponge roller, not a nap roller.

If you’re applying it to non-horizontal surfaces, you’ll want to wait a few minutes for the material to thicken so it doesn’t drip. If the coating does sag, you must sand it for a smooth finish.

Apply at least 60 mils to ensure a uniform film. Use a mil gauge to test the thickness of the layer.

If you plan to apply additional coats, you must stay within the recoat window. VFI-2519 has a recoat window of 4 hours at 50 mils. If you apply it at a greater millage, it may affect the recoat window. If you miss the recoat window, you must sand the coating before applying another layer.

Setting up to Test VFI-2519 for Recoat Window

While on the topic of recoat windows, we wanted to explain how we would test for this property. The test method is arbitrary, so there is no industry standard.

A recoat window is the time after an initial coating has been applied that a secondary coating can then be applied, ensuring a strong bond between the two layers.

For the test, we mixed up small batches of the VFI-2519. To ensure we had a sufficient amount of material, we used 206 grams of Poly (B side) and 60 grams of Iso (A side) by weight.

We applied the coating to a pre-sanded board of VFI-2538 70 D EPS Form Hard Coat, which is a similar urethane hard coat. We spread the brushable coating onto the board at 50 mils thick with a square film applicator.

The temperature was 75°F with a relative humidity of 70%. Because the humidity was so high, we immediately noticed bubbling in the mixing cup and the coating as it started to cure on the surface. This wouldn’t affect what we were testing for, but it is something to avoid if you want a smooth, bubble-free finish.

We didn’t think we would get good results if we tested the material in half-hour intervals at the start, so we applied a second 50 mil coat after the first had cured for 1 hour and 2 hours. These second coats were applied at 79°F with a relative humidity of 66%.

We also coated a second board to test in half-hour intervals from 2.5 to 4 hours. The first coats were again applied at 50 mils with the square film applicator. The temperature was around 71°F with a relative humidity of 70%. With the humidity still around the same as the first board, we noticed bubbling in the coating again.

We then applied another 50 mils on top of the first coat after it had been curing for 2.5 hours, 3 hours, 3.5 hours, and 4 hours. Note: A 5-hour test was also tacked on since we had room on the board.

The temperature was between 70-71°F with a relative humidity between 70-76%. The new layers also started to bubble. Increased bubble formation may have occurred because the previous coating was still off-gassing as well.

Testing Adhesion for Hard Coat Recoat Window

We left the boards to cure for the rest of the day before we could prepare them to test their intercoat adhesion for the recoat window.

The surface was sanded before we applied dollies from the Elcometer 106 Pull-Off Adhesion Tester to each test section with an epoxy adhesive. We left the adhesive to cure for 3 days to ensure the dollies were firmly attached to our testing surfaces.

With the dolly cutter (bimetal hole saw), we cut around the base of the dolly so lateral bonding wouldn’t interfere with the test. We then removed the excess coating and adhesive shavings.

The base support ring was placed over the dolly to ensure a flat surface. The tester claw was then clamped onto the dolly, and the wheel on top of the tester was tightened to apply a perpendicular force that would pull the dolly off. The indicator on the tester retains a PSI value at which the coating separates from the surface.

Results of the Recoat Test

We wanted to see two things from the pull-off test:

• An acceptable PSI for tensile strength
• The two coatings still adhered together

If we only saw one layer of coating and/or a low PSI, that would be a failure.

The 3.5-, 4-, and 5-hour samples failed at 200 PSI, while the 2.5- and 3-hour samples failed at 400 PSI. The 2.5- and 3-hour samples maintained intercoat adhesion, which was a success. While the 5-hour sample failed at 200 PSI, it maintained intercoat adhesion.

That’s how we determined that a second coating could be applied at 50 mils up to 4 hours after the first. We also determined that reapplication 1-2 hours after applying the first coat is not recommended. The recoat window can always change depending on the film thickness, application surface, and temperature.

Applying VFI-2519 after 4 hours will compromise the adhesion between coats. After this time, the initial coat would have to be sanded and/or primed before another could be applied.

Contact VFI if you need more assistance when applying multiple coats of a brushable hard coat.