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December 18 - December 23, 2016

Do you need to wait for the mortar to cure before repainting a brick surface after repointing and power-tool cleaning? If so, how long, and how do you judge when it is safe?

Selected Answers

From Jaime Molina of Primary Materials Inc. on December 22, 2016:
All cementious mortar will release free lime and other minerals while setting. These minerals will ride piggyback on the escaping moisture and move to the surface. The film that will form on the surface is called efflorescence, and paint or coating will not adhere properly to it. The conventional wisdom is to wait for 28 days before painting to allow for the moisture and the minerals to migrate to the surface – where they are washed off before coating the mortar. However, various techniques have been developed to treat the fresh mortar to stop efflorescence and allow painting shortly after the mortar is surface-dry. Most of these are water-soluble siliconates that will react chemically with the lime that is trying to escape and will form a polymer in each and every pore in the concrete near the surface, but no on it. This plug allows the small vapor molecule to continue to pass, but will hold the lime and minerals. It will also seal the surface to the ingress of water. There are several products that do this function and each has its own proprietary system.

From Warren Brand of Chicago Corrosion Group on December 21, 2016:
It depends on the type of mortar and paint. There are systems which are designed to go "wet on wet." You  need to check the data sheets of both the mortar and the paint.

From Wayne Yancey of CallisonRTKL on December 20, 2016:
Wait 7 days.

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