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Ask the Expert Question-and-Answer Archive

by Ron Joseph

March, 2006

Smelly Latex Paint on Concrete Blocks

Q. About two months ago volunteers in our community painted the previously painted concrete block interior walls at our community center with latex primer. Though the walls had been previously painted, they still soaked up a lot of paint. We have not applied the final top coat, so the primer has been exposed to a cold interior for several months this winter. Right after we painted there was a dampish odor, but now it has turned into a terrible strong sour smell that is almost intolerable. Do you have an explation for the odor and its increasing intensity? If we put on a top coat, will it go away? Should we treat it it any way before applying a top coat.

A. Waterborne paints usually contain biocides to prevent bacterial growth; however, you wrote that the concrete "soaked up a lot of paint" (and consequently also water) and "the primer has been exposed to a cold interior for several months this winter." You are probably smelling the rancidity of bacteria or mold.

I don't have a definitive answer for you, but in my opinion by applying a topcoat over the entrapped moisture, you will only aggrevate the problem by preventing the remaining moisture from evaporating out of the concrete. I suggest that you use lots of warm air (from heating fans) to drive out all remaining moisture from the concrete blocks. I don't know if this will totally solve the problem, but my guess is that it will help get rid of, or at least minimize the odor. Ideally, you would allow the concrete blocks to soak up a solution of bleach to kill the bacteria, but I don't think that is practical because you will need to remove the primer, and then might have problems applying the primer again at a later stage. If you completely dry out the concrete and if the rancid smell disappears, then you can consider applying the topcoat.

At the worst you can try to wash a small section of the painted walls with household bleach and see if that makes a difference. Before applying the topcoat you will need to rinse off all unreacted bleach with fresh water.

I empathise with you and your fellow volunteers, but this is a problem that might be difficult to eliminate.

Best wishes,

Ron Joseph

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