When it comes to exterior paint, we get a lot of questions about paint failure. Most of these failures are on concrete block or CMU (CMU=concrete masonry unit) buildings. Large CMU commercial buildings must be painted and repainted correctly in order to get a long lasting, protective paint job. First, I’ll go over how to correctly paint and repaint these types of buildings.
New CMU Prep
Most painters who are experienced will tell you that it takes 7 to 30 days for the masonry joints to cure before you can paint them. This is not negotiable. The PH in the joints cannot exceed a PH of 13 and even that is pretty ambitious for even the newest technologies in high PH paints. You want your contractor to perform a PH test before they paint your new CMU. This test is simple and only requires distilled water, a spray bottle, and PH test strips. We usually look for a PH of 12 or less to ensure the paint is not degraded due to PH burn. This is the most common reason the first paint job usually fails. The signs that your building was painted too early is that there are the white streaks coming out at the masonry joints of your concrete block and the paint on the joints is bubbling or peeling.
Priming is another important step that is a major component of most paint failures. Your contractor needs to use a high quality, acrylic block filler. Let’s discuss the three types of primers for cement block. The first is in the never ever use this list. These are the old calcium based fillers that are dirt cheap. As a contractor, we can purchase these for under or around $10 per gallon and, at only 60 square feet per gallon, many are tempted to go this route to save money or to shave a little off the quote too get the end user to hire them. Don’t do it, you will regret it. These fillers are not robust and will soften if they ever become wet again. If you building develops hairline cracks or gets damaged, water will penetrate the cracks and start to soften these primers. This causes them to degrade quickly and the damage to the paint below the damage will need to be removed before repainting. The trade names of these fillers are Sherwin Williams Prep-Rite Block Filler and PPG SpeedHide Block Filler. Avoid these at all costs! The middle of the road solution, which we highly recommend, is a hybrid type that is acrylic based. This type costs in the upper teens and is sold under the names Sherwin Williams Heavy Duty Block Filler or PPG SpeedHide Hi-Fill Block Filler. The third type, if you budget can afford it, is the best type and would come with our best warranty. These products are sold under the names Sherwin Williams Loxon Block Surfacer and PPG Perma-Crete Concrete Block & Masonry Surfacer/Filler. These last two types can be used on unpainted CMU or previously painted CMU. The first type will not adhere to existing paint, so don’t wast your time!
The final thing to make sure of when you are painting a new CMU building is to ensure you have full coverage of the CMU. You are aiming at zero pinholes or pores. You should insist on photos or an inspection by you or a knowledgeable representative before any topcoats are applied. Many of the larger box stores will actually send their construction managers on to the roof to inspect and get photos of the top angle of the CMU to ensure and document that all pores were filled completely. At Indiana Painting we both document with photos and record how many gallons were used to prime a building to both visually acknowledge and to perform a sanity check to make sure you are getting a top quality paint job! Paying close attention to the quality of this work will give you many returns for your time in the coming decade and will keep you from having a difficult situation to deal with!
Existing CMU Prep
How can you be sure your existing CMU is in good shape? You can’t always tell with a simple visual inspection. Our Project Consultants will run a feeler over your walls in various areas to listen for paint that has become disbonded. How does this happen? Some budget or inexperienced contractors will take a shortcut and paint over paint that is already starting to fail. Many can get away with it for a year or two because acrylic paints are great at bonding to themselves. This will create a “shell” until it comes under stress and breaks away from itself. If we identify any failing areas, it will change how the building must be prepared for new paint. If everything is in good shape, a simple pressure wash job to remove any atmospheric contamination and faded/degraded paint. If you have failing paint, we use a 5000+ psi pressure washer and a zero degree rotating tip to remove as much failing paint as possible. This is followed up by a scraping any remaining paint to ensure a sound surface to adhere to with no loose underlying paint. If your building has more than 20% failing paint, we recommend priming at least the areas with the worst damage, if not, spot priming is sufficient.
Finish Coats
We always recommend a good, quality paint for your building. If this is your first paint job, we recommend a high build product by PPG called Perma-Crete High Build Masonry Coating. This is not an elastomeric paint as it is acrylic and retains breathability. We would recommend you stay away from elastomeric paints here in the midwest. We see too many failures and have repainted countless faded and damaged building that have used them over the years. Our favorite product for new and repaints is either PPG Acri-Shield or PPG Permanizer as it has excellent fade resistance and is very durable. We also recommend you don’t stray from the recommended two coat system on exteriors and, if your budget allows, put a full third coat on to ensure coverage and durability.
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