Vladimir Popovac, a mason in Sonora, California, replies: Mortar achieves around 90% of its hardness and bonding power within the first three days of application. After 30 days, mortar’s hardness reaches closer to 99%. So needless to say, it’s easiest to clean mortar from brick in those crucial first few hours after it has set up.
I try to clean mortar from brick or stone as soon as I can brush the mortar without smearing it in the joints. At that point, a dry brush and a sponge damp with water are all that’s needed. However, cleaning mortar off brick even years after application is still possible, and muriatic acid is still the agent to use.
Muriatic acid is a form of hydrochloric acid that dissolves mortar. Be sure to follow the instructions on the container closely. Typically, those instructions say to begin by mixing a 9:1 (10%) solution of water to acid. The acid should always be added to the water and not vice versa.
Next the brick is wet down with water, and the acid solution is applied with an acid-resistant brush. The solution should sizzle and fizz as it dissolves the mortar. Leave the solution on for a few minutes, and then rinse the brick with water. It’s important to rinse between applications because the acid solution will begin to dissolve the mortar in the joints as well as the brick itself.
If the mortar you’re removing doesn’t come off on the first try, repeat the process until it does. As the mortar bond weakens, use mechanical and abrasive force in the form of a chisel, a scraper or an abrasive pad. If you need to speed up the process, gradually increase the amount of acid in the mix. When all the clumps of mortar are removed, you usually will be left with a stain that looks like a faint shadow after the brick dries. At this point, apply a coat of masonry sealer, and that shadow should disappear. There are many sealers on the market, but one that I’ve had good luck with is Glaze ’N Seal (800-486- 1414). As a word of caution, be sure to wear eye protection, acid-resistant gloves and old clothes when cleaning with muriatic acid. Fabric and flesh don’t offer nearly the resistance to the acid that the mortar does.