Many people say that concrete must cure for 28 days before it can be painted. The 28-day figure comes from the fact that at that point concrete reaches its maximum compression strength. That said, concrete can indeed be painted sooner than 28 days. Below are other factors to keep in mind for new concrete projects in either new builds or renovations.
Measure pH and Moisture Content
The best thing today in estimating when to paint concrete is measure the pH level and moisture content of the curing concrete. You can test pH digital gauges, pencils, and, for larger projects, a rainbow indicator can be used. This is sprayed onto the concrete, and when it dries it will gauge the pH by way of its color reaction to the concrete. That color is then matched to a color chart. Strips that are similar to those used in testing a pool's pH can also be used.
Moisture tests are simpler. You can tape a plastic sheet to the concrete and then use a moisture meter to measure the concrete's moisture. It should be less than 5% before coating application. One thing to keep in mind with both of these tests is that multiple tests might be required, especially when it comes to moisture, and particular when the commercial space is of considerable dimensions. These will then need to be averaged to determine if it's optimal for coating.
Other Factors
On previously painted concrete surfaces, the existing coating should be examined to determine its dry film thicknesses. This helps to determine if the coating is sound. Elastomeric coatings can be recoated with acrylic finishes. Laitance should be removed, as should Efflorescence, which is a powdery substance that leaches out of the concrete. For both existing projects and new builds, be sure that there are no cracks or other damage to the concrete. This will ensure adhesion.
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