Swipe to view slides
  • Etching from vinegar-based hot sauce.
  • Light etching.
  • Failed sealer and etched concrete sink.
  • Honing off the failing sealer.
  • Grouting the sink with color-matched rapid hardening grout.
  • Honing off the hardened grout.
  • Acetone wipe-down to clean and dry the concrete
    before sealing.
  • Aluminum foil masking on exposed cabinetry.
  • Wiping on the UV-cured urethane sealer.
  • UV curing demonstration.
  • Fully repaired sink.

Question:
One of my customers has white spots in their concrete countertops after cutting lemons and limes on the surface. They've had no success removing the stains and want my help. What's the best method for removing acid stains?

Answer:
White spots from lemon juice and other acids are actually not stains at all. Instead, they are etching where the acid dissolved the cement paste and left white carbonate deposits.

Unlike stains, etching is physical damage to the surface. An acid (either citric acid from fruit or acetic acid from vinegar-based foods) reacts with the cement paste and any calcium-based aggregate, gradually dissolving the materials and leaving a rough depression and a light-colored appearance. Repairing an etched area often involves honing or polishing out the white spot, grouting to fill exposed pinholes, and resealing to restore the finish. As you can imagine, this is a lengthy process that takes skill, practice, and the proper tools to do well and is not something a homeowner should tackle.

For very light etching, often a fine-grit diamond hand pad is adequate. Start with an 800-grit pad and see if it's effective. If you seem to be doing a lot of rubbing with little result, then switch to a slightly coarser 400-grit pad. The goal is to remove the visual damage without substantially altering the concrete.

Larger etched areas might need to be honed or polished using an electric polisher. Since repairs are likely going to be done in a home, dry-polishing pads are more practical than wet-polishing diamond pads. Since dry polishing can generate harmful dust, prewetting the concrete with a tile sponge can virtually eliminate dust, leaving behind only a thick paste. Once the etch marks are removed, you can reseal the surface.

Deeply etched areas often need to be regrouted to replace the cement paste that was dissolved by the acid. This will restore the appearance and surface texture to like-new conditions. Once the grout is hard, you can hone it off with fine-grit diamond pads (usually 200 or 400 grit) and then reseal the newly restored concrete.

The following photos show the sequence of how I restored a pair of etched concrete sinks. The reactive sealer had completely worn away after a year of use, leaving the bare concrete vulnerable to mild etching.

The first step was to hone off the remaining sealer and prepare the concrete for regrouting. Usually grout used for concrete countertops is made with portland cement. However, that can take a day or longer to harden. Since this was a repair, time was of the essence. I used CSA (calcium sulfo-aluminate) cement instead of portland cement, so I only had to wait about an hour before the grout could be honed off. (Ordinary grout usually takes 16 to 24 hours to get hard enough.) With the CSA-based grout, I am able to do two or three applications in a single day, if necessary, to get the concrete surface as smooth and even as possible.

I chose to seal the sinks with a different sealer. To return the sinks to use as fast as possible, I used a urethane sealer that cures upon exposure to ultraviolet light. Because this type of sealer needs the concrete to be very dry to ensure good bonding, I let the bare concrete dry overnight. Then I wiped it down with acetone, which removed any residue and helped to draw moisture out of the concrete. Because the UV light is very powerful and could have discolored the homeowner's hardwood cabinetry, I thoroughly masked everything with old sheets and aluminum foil.

I also took care in applying the sealer, because I didn't want to get it on the top of the countertop or the on the glass tile backsplash. So I chose to wipe it on rather than on applying it by brush or roller. Wiping leaves a very thin film of finish, so I had to cure each coat, lightly sand the surface, and then clean the dust off with acetone.

It took two days to do the full repair. The first day involved a few hours of honing and grouting. On the second day, it took me about an hour to seal both sinks with four coats of finish. As soon as the last coat cured, I was able to turn the water on and use the sink.

So far these finish coats have withstood the test of time. After a year and a half, there is no water penetration and no signs of etching or delamination.

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