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Q.We blocked over a couple of basement windows in a stuccoed stone foundation wall. What’s the best way to patch the stucco? Can a simple bagged mortar mix be used to cover the block, or do we need to use stucco?

A.Steve Thomas responds: Several things make this type of job very challenging: It can be difficult to finish the wall on the same plane as the existing stucco. You’ll undoubtedly wind up with shrinkage cracking around the perimeter of the old opening. And most challenging of all, it’s extremely difficult to match the color and texture of existing weathered stucco with a new mix.

It’s likely the existing coating on the foundation is a multi-layer cement parge coat, and not true stucco. True stucco contains white waterproof cement, lime, and silica sand, and is applied over metal lathe. Each coat is mixed to different strengths, starting with the scratch coat, which has the highest cement content for the highest strength. Each layer outwards gets increasingly weaker, with the finish coat being the weakest and least brittle to minimize cracking.

Instead of using a bagged mortar mix, repair the parge coat with a product such as Monocouche (Parex, P.O. Box 189, Redan, GA 30074; 800/537-2739). This can be applied in one coat (just add water). It’s incredibly strong; you can build it up to about 21/4 inches thick without it checking or cracking. Also, like true stucco, Monocouche has a white base, so it can be tinted with iron-oxide pigments to closely match the existing finish.

Steve Thomas is a former stucco contractor in Columbus, Ohio.