With ordinary paint moisture can usually find a way in but it is difficult for the water vapour to escape. This will cause your paint to flake or delaminate. High moisture substrates, or the underlying layers beneath your paint can cause even the highest quality paint to fail. So, when painting masonry always use a breathable paint. This would require that the paint has a vapour permeability of at least 75 perms.
If the masonry surface you are considering painting has a white flaky substance on it, you can’t just scrape it off and throw on a coat of paint and expect it to last. This substance is called “efflorescence” and is a warning sign that you have a moisture problem. Masonry products are very strong, but they are also very porous. As moisture migrates through the masonry it picks up and carries minerals with it. When this moisture arrives at the outer surface of the masonry it evaporates and leaves all the minerals on the surface. In a situation like this you must properly seal the surface before you paint, or your paint job won’t stand the test of time.
There are ways of testing concrete for moisture content. When concrete is mixed it has a lot of extra water so it can flow into place. This extra moisture needs to evaporate before any new concrete surface can be painted. One way to test concrete is by using calcium chloride. You start by cleaning a spot on the concrete and record the date and time. Then seal the pre-measured container of dry calcium chloride to the concrete using tape. The more moisture this substance absorbs the more moisture is in the concrete. This is important because some paint manufacturers won’t back their warranties above certain levels. One problem with this method of measuring moisture is that it only measures the moisture at the surface of the concrete.
Another method of testing moisture in concrete that is more reliable is the Relative Humidity test. It tests the moisture levels deep into the concrete. You start this test by drilling a hole at least to a depth of 40% of the thickness of the concrete. After cleaning the hole, you put a humidity sensor into the hole. You should be safe coating the concrete if the relative humidity in the hole is less than 70%.
Another factor to understand when coating concrete is pH values. A neutral pH is a value of 7. Anything above that is alkaline and anything below 7 is acidic. This test is completed by sanding down and cleaning a test patch to remove any old coatings. Then using a pH testing kit measure the pH value of the concrete surface to ensure that the surface is not to alkaline for your coating specifications.
Understanding the moisture content of the surface you are painting is important to the longevity of your paint job.