Pinholing Caused By Acid Stains

Question:
After acid staining my countertops, the finished surface is full of pinholes. Why does this happen and how do I fix it?

Answer:
Acid stains rely on the acid component to generate the color reaction. In the process, they etch the surface and dissolve some of the cement paste. The degree of etching and dissolution depends on the strength of the acid stain and the duration of exposure.

Precast concrete countertops tend to be more susceptible to acid-stain pinholing than cast-in place countertops because the concrete near the surface of precast concrete has more air bubbles in it than a hard-troweled finish. Troweling reworks and smears the cement paste, or cream, eliminating the trapped bubbles that would otherwise be exposed after acid staining.

With cement-cast finishes (a precast technique), the hardened cream layer is the finished surface. Often little or no honing is performed on this cream layer to prevent exposing the air bubbles that lurk just below the surface. However, this makes the cast finish more susceptible to acid-stain pinholing because of the thinness of the cream layer and the unfilled air bubbles that lie below. Honed or ground concrete, on the other hand, removes the cream layer and exposes the air bubbles. The open bubble holes are then filled with a cement slurry. The resulting surface has less exposed cement paste and a lower probability of pinholes becoming exposed due to etching.

Since you’ve already acid stained the concrete, you won’t be able to fill in the pinholes with slurry without altering the appearance. A common technique for dealing with the problem is to apply a coat of sealer to the concrete and then to fill in the holes with a tinted epoxy. This first coat of sealer prevents the epoxy from streaking or discoloring the concrete, since epoxy tends to darken the concrete more than most sealers do.

To fill the holes, use an epoxy formulated to set quickly (within about 5 to 10 minutes). To tint the epoxy, mix in dry concrete pigments or use colors made for tinting epoxy. Sand off any excess epoxy after it has hardened. Take care when sanding so that the acid staining isn’t removed. Acid stain color is literally skin deep, and sanding will remove it.

This technique works best for concrete that will be sealed with a coating, such as an acrylic, epoxy or urethane. But it can also be used successfully with polished concrete, densified concrete and concrete treated with a water repellant. Practice is important when applying epoxy to these finishes to prevent spotting.