When it comes to decorative concrete, there’s a huge difference between interior and exterior projects. While interior concrete is protected from the elements, exterior concrete must withstand harsh UV rays, wet weather, heavier traffic, and chemical attack. These factors cause most of the problems with exterior concrete finishes, and account for a majority of callbacks from dissatisfied customers. Although it’s easy to blame surface defects on the elements, many contractors unwittingly contribute to them by treating interior and exterior concrete the same. They fail to realize that the techniques used to install one do not necessarily transfer to the other.
Adequate release removal is crucial
The most glaring example is the growing trend toward using tinted liquid release to antique exterior stamped concrete. This antiquing technique was originally developed to obtain a two-tone effect on interior stamped concrete where using powdered release would create a big mess. Today, however, it is increasingly being used to expedite exterior projects by cutting down on cleanup time.
Unfortunately, in the rush to seal the job and get paid, contractors sometimes skip the crucial step of cleaning off the excess release residue. Many simply wait for the liquid release to evaporate, and then seal the concrete without removing the residue. The contractors are not necessarily to blame. Many of them have been taught this method, and it is the only way they know how to do it.
Tinted liquid release is made in the field by mixing a small amount of powdered antiquing release with clear liquid release. This mixture is then sprayed onto the stamped surface where it runs off the high spots and puddles in the depressions. The liquid then evaporates and the colored powder release is left behind to be locked in with the seal coat.
This method may work fine on sheltered interior stamped concrete, but can cause big problems outdoors. Unlike stamping with powdered release, little to none of the antiquing color is actually embedded into the surface cement paste, but is simply lying on top. When the acrylic sealer — which is notoriously soft and degrades quickly — wears or flakes off, it takes the antiquing color with it. This results in a blotchy surface devoid of secondary color. The original color cannot be restored simply by resealing the bare spots. To match its surroundings the surface must first be re-antiqued, which is often done by sealing in more powdered release. And the cycle repeats itself.
Of course, the same sealer delamination and blotchiness can occur with powdered antiquing release. The issue with either method becomes, “Was enough release removed from the surface before sealing?” Because both powdered and liquid release are bond breakers, they can also break the bond between the sealer and the slab. Sealing atop too much residual release is the same as painting over dirt. When the paint (sealer) peels off, the dirt (powdered release) is stuck to its back side. The peeled surface (concrete slab) is a bit cleaner (blotchy) as a result.