While the term is more commonly used in reference to applying drywall mud to a surface in preparation for finishing, skim coating is also used in concrete work, as well as with ceramic tile and natural stone installations. Applying a skim coat of cement is useful for giving new life to concrete surfaces that have pitted or cracked over the years, but it is also useful for covering mason block walls in preparation for a ceramic tile or natural stone installation. While you can buy pre-mixed skim coat materials, you can also make your own.
Determine the area for which you are going to skim coat. If you are covering an existing concrete floor and are simply covering up old cracks or helping to level out dips and valleys, you can use a simple concrete and water mixture. If you are skim coating walls, such as with bricks or masonry blocks, you can add a little lime into the mixture to create a more sticky compound that more readily sticks to the surface, allowing for ease of use.
Mix your skim coat mixture with a low-speed drill. A high-speed drill will whip air bubbles into the mixture, which will pop as the skim coat dries on your chosen surface, leaving pin holes behind. Add water in small amounts and aim for a consistency just this side of thick peanut butter. The goal is to have it spreadable, but not soupy or runny. Let the mixture set for several minutes after mixing it the first time to let the water completely soak into the concrete, then mix it an additional time before spreading.
Add lime into the mixture. The amount of lime is somewhat irrelevant, but as a general rule you can add a couple of handfuls of lime to a five-gallon bucket of skim coat mixture. The more lime you add, the stickier the skim coat mixture will be. Some experts choose a 5-to-1 mixture of cement to lime. Otherwise, you can mix the skim coat batch in the same way you would with a normal cement and water base.