Published 08/01/2004

Repainting Concrete

Question: Our firm recently was contracted to do a repaint of an existing concrete structure.

Liquid Coating

Consultant

Question:

Our firm recently was contracted to do a repaint of an existing concrete structure. I know very little of the history of the building other than it stood for about 25 years as a plain grey raw concrete tilt up structure. About two years ago, someone decided to paint the exterior. I have no idea what method of surface prep was used on the original paint job. My dilemma is, what we figured to be around a 40 man-hour pressure wash job to remove several areas of loose flaking paint has turned into 60 man-hours and the paint just keeps coming off. Now, the existing paint seems to be failing everywhere. I have collected some of the paint chips and it looks like a primer of sorts was used topcoated with a thin coat of exterior latex. Not very much thickness at all. On the backside of the primer coat, there is a lot of chalking and even some small bits of concrete sticking to the primer.

I am beginning to think that the concrete panels may be unsound. It does have some cap flashing on the top of the walls. I am bewildered as to what to recommend to the building owner as to what is causing the failure. And of course, what can be done to fix the problem and keep within the budget, they have to spend.

Is there anywhere I can have the paint chips analyzed in my area to determine what is the cause of failure. I certainly cannot warranty the paint job without knowing what is causing the original paint to fail and then come up with a solution. One really tough part is the man who gave me this lead was the painter who painted the building originally and now he is my local paint rep. Go figure. Any suggestions or assistance will be greatly appreciated. C. P.

Answer:

Since the Painting Clinic usually answers questions concerning finishing products, this question is really outside its scope. However, since buildings in which products are finished often need to be repainted, I decided to respond. This is, after all, a full service column.

Painting concrete structures is difficult for a number of reasons. One important reason is that the surfaces are not homogeneous. Another reason is the presence of loose and powdery materials on the surface. I must say, I am surprised that there is still loose material present after 25 years of weathering, surface preparation and painting.

Ask the man (your paint rep), who painted the building two years ago, what surface preparation and paints he used. Other than loose concrete particles, I have no clue as to the composition of the loose surface material. Loose concrete particles are generally washed away during weathering. However, I can tell you that any loose particles must be removed before repainting. There are testing laboratories in metropolitan areas that are able to tell you the composition of the loose surface material. Pressure washing may not be the best way to remove the paint and loose material. Instead, you may consider using a light sandblast.