There are many patching techniques used by installers throughout the country. While many installers would state unequivocally that their technique is flawless, just as many are unaware of the problems created by improper use of patching compounds. The following is an attempt to clarify the proper use of cementitious patching compounds.
Read the manufacturer’s instructions
Each manufacturer will have their own specific recommendations regarding the use and mixing of their patch, and the approximate drying time of the same.
Measure both the water and the patch
To ensure a proper mix without over-watering, the powder and water should be mixed. The measuring allows for the setting and compressive strengths to work as the manufacturer intended. Over-watering a mix will lower the compressive strength and increase the shrinkage of the final set.
Mix the patch in a plastic container
Use a plastic container to mix the water and powder. Never mix the patching compound directly on the surface of the floor. The water is always placed into the container first, followed by the powder. The reason for the placement of the water first is to obtain a smoother, lump free mix.
Use a drill to power mix
The use of a power mixer will yield a smoother, thinner mix. The speed of the power mixer will break the surface tension of the water, which will thin the mix to a thinner state without the addition of water. This will create a mix that is easy to work and will tend to go further.
Pre-wet the area to be patched
If the substrate is dry and tends to absorb the water out of the patching compound too fast, you can resort to pre-wetting the area to be patched. The pre-wetting will stop the substrate’s ability to absorb the moisture out of the patch, allowing it to remain trowelable longer. Plus, pre-wetting will also allow for a stronger bond. Never add more water to the patch. The addition of water to the already mixed patch will compromise the mix.
Keep the mix stirred
By keeping the mix stirred, it will keep the mix in a thinner state. The occasional stirring will keep the surface tension of the water broken.
Trowel the finish to a smooth coat
Keep the trowel at about a 60º angle and move the trowel in long smooth strokes.
Keep the trowel from leaving a ridge
Be sure to keep the trowel from allowing the leading edge to leave a ridge. This can be done by slightly rounding the trowel corners.
Allow the patch to thoroughly dry
While some manufacturers indicate that you can go over their patch while it is still damp, I prefer to allow the patch to dry.
Sand the finished surface to be sure that all the irregularities are removed
Use a sanding block to sand the patched area. This will remove any irregular humps and bumps that could show through the finished floor.
In the October 2019 issue of FCI, INSTALL’s executive director John T. McGrath, Jr. and instructor David Gross share key lessons to ensure concrete polishing jobs are done right the first time, every time.