Ask the Expert Question-and-Answer Archive

by Ron Joseph

April, 2003

Painting New and Aged Concrete

Q. Do I have to acid etch new concrete that has cured for 6 months and has never been sealed or polished?

A. You don't say what you intend to do with the concrete, I assume that you are going to apply a paint over it. Typically one would etch new concrete to help neutralize the alkalinity of the surface in order to help insure good adhesion of the coating.

If the concrete surface has been subjected to grease and oil, then a strong detergent would be in order to insure the surface is clean of these contaminants prior to application of any paint.

Concrete typically has a porous surface and will allow a paint to adhere well to it, if it is good and clean, however some floor surfaces have been trawled to a very smooth finish and may need the aid of an acid solution to etch the surface to create a profile in which the paint will adhere.

Be certain that the surface is thoroughly cleaned with a good detergent. An acid etch at this point will help to insure a good clean surface and aid in the adhesion of the paint. One of the most common acids used for this purpose is muriatic acid that you can purchase from a swimming pool supplier. If you decide to use this you MUST insure that you are properly protected with goggles, acid-resistant gloves and proper clothing so that you don't injure yourself. For similar reasons, please insure that you keep others away from this process. After performing the acid etching operation, wash down the concrete with fresh tap water to remove any remaining unreacted acid. Allow the concrete to dry completely prior to any application of paint. Depending on the weather you might need to wait several days before applying the coating.