How to Spray Apply GFRC

Question:
What sort of equipment do I need to spray apply glass-fiber-reinforced concrete into countertop forms? And how many coats of material should I apply?

Answer:
GFRC is a lightweight, high-strength cement mixture that contains alkali-resistant glass fibers. (Read “The Benefits of Using a GFRC Mix for Countertops.”) Typically the material is applied in two layers. The first layer, or face coat, usually has no fibers in it and is thin, often only about 1/8 inch. The second, or backer layer, contains the fibers and is applied to achieve an overall thickness of 3/4 to 1 inch.

The face mix and the backer mix are applied at different times, so it’s important to ensure that the general makeup of the two mixes is similar. Water-cement ratios and polymer contents should be the same to prevent curling.

A common and inexpensive method for spraying the face coat is to use an inexpensive hopper gun powered by an air compressor. Hopper guns are often used to spray cementitious overlays or other knock-down surfaces. The combination of a hopper gun and a 60-gallon air compressor can cost as little as $400 to $500. However, you won’t be able to use a hopper gun to apply the backer mix because of the heavy dose of fibers it contains. Hand placement or conventional pouring of a self-consolidating mixture will be required. To spray the backer mix, you would need a much more expensive premix spray gun or concentric chop gun, a pump and a compressor—a setup that can cost $10,000 or more.

Once you spray apply the thin face mix into the forms, allow it to stiffen before applying the backer mix. This prevents the backer mix from being pushed through the thin face mix. When applying the backer layer by hand, place it in layers 3/8 to 1/2 inch thick and compact each layer to remove air voids and ensure good bonding with the face mix. Specialized spring rollers are the tool of choice for this job.