Skim coating is the process of applying a layer of muddy plaster compound to rough or damaged ceilings and walls to either smooth or repair them. Both professional plasterers and Do-It-Yourselfs (DIYs) can use this method to repair entire walls; to smooth out cross-hatching on dry wall paper, fill deep scratches or holes; or to replace water-damaged portions of drywall. The skim coating process generally takes at least two layers of compound. After all the coats are dry and smooth, the wall or ceiling has to be sanded before it can be painted or wallpapered. This process can also be used to give concrete a smooth surface.
Tools and Materials
The main material needed for skim coating is joint compound. This is a thick but spreadable substance that dries and hardens over time, maintaining the drywall or plaster's integrity and continuity. It generally comes in a powder that has to be mixed with water, or in ready-made mixtures. Both work well, but the powder version often dries faster than the ready-made one.
Tools needed for skim coating are a mud pan to mix the compound in and a taping knife or trowel to apply it to the walls. This can also be done with a roller brush if the compound is very thin. A blister brush or paintbrush may also be needed for smoothing dried joint compound out. Some people also use a square carrier called a hawk as a palette to hold compound and move it around the room more easily.
Skim Coating Process
If ready-made compound isn't being used, then the compound will have to be prepared first. This is done by adding small amounts of water to the powder and mixing it until it becomes liquid, but not watery. Consistency-wise, it should look somewhat like cake batter. If it is too watery, and doesn't stick to the trowel or blade, then more powder is needed.
Before skim coating, the wall or ceiling should be prepped by lightly sanding it and scraping off all the loose pieces, and cleaning it with a vacuum or damp cloth to remove all the dust. If there are any joints open, then they should be taped. Once this is done, about 1/8 inch (about 0.3 cm) of material should be smoothed over a small area of the wall. Extra compound should be scraped off of the knife and gently dragged across the area to smooth off any excess remaining on the wall. If the compound is already hard, a blister brush or paint brush can be used to put a little bit of water on the walls to soften it before smoothing.
Once the first layer has dried, any remaining ridges or bubbles should be scraped off with the knife. Any rough areas should be lightly sanded, and then cleaned up as before. A second coat can then be applied, working in the opposite direction as the first. Once all the layers are dry, the wall or ceiling should be sanded before wallpapering or painting.
Skim coating should be done in about 3 feet square sections, so that there's time to smooth the compound before it dries. If a whole wall is being done, then it's best to do it in halves, dividing walls horizontally down the middle. When doing the top half of the wall, compound should be smoothed down from the top, and then the bottom half can be by smoothing compound up from the floor. The knife should be scraped clean or wiped clean with a damp rag if it starts to get compound built up on it. Additionally, all materials should be cleaned well immediately after use, to prevent mud from hardening and coating the tools.
The process for skim coating concrete is much the same; however, the material used is not joint compound, but rather, a solution designed for use with concrete. It's sometimes called a skim coat, a rapid coat, or a thin coat. Additionally, the concrete floor needs to be damp before applying the skim coat, so that it will adhere well, so a small pump sprayer should be used to dampen sections of floor before application. The method for applying and smoothing the material is essentially the same as for walls and ceilings.