Avoiding Etching Spots from Acid Staining

Question:
My polished concrete countertops show spotty etching after acid staining. What’s going on?

Answer:
Acid stains will almost always etch the concrete surface to some degree. But polished concrete (assuming you are polishing to about 3000 grit) will accentuate variations in surface texture caused by the acid.Usually when you see these spots more in some areas than others, it means the acid was more concentrated in those etched areas or the acid pooled in those spots and was spread thinner in other areas, so the degree of etching varied. Either way, the etching is accentuated by the sheen of the highly polished surface.

When acid stain is first applied to concrete, the acid concentration is at its peak and then decreases as the stain reacts with the concrete. Thinner applications of acid stain loose their concentration faster than heavier pools of stain, so it’s important to apply the stain evenly to prevent spotting, and therefore etching. If acid stain is applied with a brush, the first areas where the brush touches the concrete will stain more aggressively than the surrounding areas unless you immediately spread out the stain.

To get the most even stain coverage without causing spots, thoroughly wet the concrete surface with a thin, even film of water. Next, dilute the acid stain so it's not too strong, and then spray apply the solution with a small pump-up sprayer, applying enough stain so it goes on evenly. The thin film of water minimizes spot etching by instantly diluting and spreading the acid stain droplets as they hit the surface and by acting as a buffer to prevent burning where the acid stain first hits.