HOW LONG A WAIT TO PAINT?

Q: We placed a concrete slab for an exterior multipurpose game court. How long do we have to wait before painting basketball-court lines?

A: Because many different paint types are available for concrete surfaces, you should follow the instructions provided by the paint manufacturer. Most literature recommends a 28-day cure, but it's not clear whether that means a 28-day drying period after the concrete has been moist cured, or a 28-day wait after the concrete has been placed. The Portland Cement Association (Ref. 1) recommends waiting 28 days to 6 months, depending on surface conditions and the coatings used on the surface.

An American Concrete Institute publication (Ref. 2) suggests that the moisture content of concrete is excessive for paint application if moisture collects at the bond line between the concrete and the paint before the paint has cured. This can be evaluated by taping a 4x4-foot clear plastic sheet to the concrete surface and determining the time required for moisture to collect on the underside of the sheet. During the test, the ambient conditions (sunlight, temperature, and relative humidity) should simulate, as much as practical, the conditions existing during paint application and curing. Compare the time for the moisture to collect with the paint curing time--a value that should be supplied by the paint manufacturer. The concrete is adequately dry if the paint will cure in a time shorter than that required for moisture to collect under the plastic sheet.

References

• "Painting Concrete," IS134, Portland Cement Association, Skokie, Ill., 1992.
• ACI 515.1R-79, "A Guide to the Use of Waterproofing, Dampproofing, Protective, and Decorative Barrier Systems for Concrete," American Concrete Institute, Farmington Hills, Mich., 1979, p. 16.