So, is it really necessary to prep concrete to receive a decorative coating?
The answer is a resounding… yes. Don’t let some products fool you. While there are a small handful of “green” or “odor free” products that will clean the surface (they mostly degrease and provide a very light etching) simply cleaning the concrete is only half the battle.
I take that back… cleaning the concrete is probably only about one third of the battle.
It makes sense: if you are going to apply a decorative concrete coating, the surface of the concrete should be clean and free of debris. Check any product label, they will all state this very clearly. But clean concrete – even concrete that has been degreased, scrubbed and pressure washed – is just the beginning. Here is why:
Concrete has a natural porosity, an ultra fine network known as the “capillary network” that is formed when the cement paste is introduced to water. As we all know, concrete hardens in the presence of water. This is known as “hydration”. Hard and durable concrete is good. But when it comes to applying a decorative concrete coating, it is the surface of the concrete that comes into question.
When a concrete surface is “floated” (the process by which latent water is brought to the surface and the aggregate tamped down to provide a smooth finish) a lot of the “cream” is brought to the top where it dries. This creates a kind of skin that is actually very attractive and smooth. But this smooth surface does not allow coatings to “key” in.
Simply put, the surface needs to be roughed up. And the best way to accomplish this is by acid etching or mechanically prepping.
Acid etching involves the use of muriatic acid (read precautions, be very careful) diluted with water. Use a common sprinkling can (the kind used in gardens) to wet the entire surface of the slab. Have a partner scrub the acid/water mixture in using a synthetic bristle broom. And keep the surface wet; acid that dries on the surface will create delam problems. Allow the acid to sit for a few minutes and then rinse with an ammonia/water mixture. Rinse very well and allow to dry.
Mechanical surface prep works in a similar fashion. This method incorporates the use of surface grinders to floor machines fitted with either diamond grinders or aggressive scuffing pads, depending on how much “roughing up” is required.
In both cases, the roughed-up surface of the concrete should feel like sandpaper. Your concrete is now degreased, washed and properly prepped to receive any decorative concrete coating.
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