If you are like most large building owners or managers you know the familiar story. When you moved into your new shiny building and it looked good – really good! About a year or two later, you start to notice it isn’t looking so good anymore. You start seeing hairline cracks and the paint around the cracks is starting to look faded and whitish. You might even have the presence of mind to get out that O&M manual to call the GC who built it or to call the architect and see what is going on with your building. The GC will most likely just shrug his shoulders and tell you your warranty expired one year after the building was built. The architect might show more sympathy, but he wonders what you want him to do about it. This is one of the most common complaints with precast buildings that we encounter. Most new build specifications on painting precast call for pressure washing, one coat of an alkali resisting primer and two coats of paint. A completely inadequate system for a 9” thick panel filled with high PH and lime that is just waiting to come out. In reality, better specifications would be costly and nobody else is doing it so that would put most ground up quotes out of the competitive range. So most new precast buildings will need to be repainted within the first 5 years of service.
So, let’s talk about the correct way to paint precast. With concrete, timing and prep is everything. Most precast is painted within six months of the panel creation date. It takes a lot longer than this for the PH levels in that much concrete to get low enough to be safe for most waterborne paints. A really thorough pressure washing is the first step. Pressure washing removes the bond breaker used to make sure the panel cleanly releases from the form. Most bond break used today is “self dissipating” meaning that, given enough time and rain, it will eventually dissolve and should be safe to paint. Never rely on this. There are still precast manufactures not using this new technology and you don’t want to chance your very expensive paint to this fate. A thorough pressure wash is a must. When we paint new build precast, we make sure we go through plenty of water and we always do quite a few adhesion tests to make sure the masonry primer will adhere well to the precast panels.
Second, you have to use a primer that will resist high alkalinity Many paint manufacturers sell two types of primer a thin type called a conditioner and a thick type called a masonry primer. We always opt for the primer type. Before we apply the prime coat, we always test for PH. Simply spray some distilled water on the panel and then use the PH test strips. These can be purchase online or at any pool store. If the PH is over 12, we always recommend holding off and trying again in a month or so. If the work must continue, we would consider applying an additional coat of primer or using the conditioner as the first coat and primer as the second coat.
Finally, we always recommend using a topcoat that is made to resist the alkali in the concrete. Our turn to products are Permacrete High Build Masonry coating from PPG or the Loxon Masonry Coating from Sherwin Williams. These products give you a pretty good chance of getting a good 10 years out of your first paint job. Some owners and architects ask for an elastomeric coating, but we do not recommend this in our area for quite a few reasons. More on that in another post.