What Is A Skim Coating in Construction?
What Is A Skim Coating?
Skim coating is a process used to repair or smooth out walls and ceilings that have become rough or damaged. It involves the application of a layer of joint compound, also known as muddy plaster, to the surface in question.
This can be done by both professional plasterers and DIY enthusiasts to repair entire walls, fill in deep scratches or holes, or replace water-damaged portions of drywall. The process typically requires at least two layers of compound, and once it is complete, the wall or ceiling must be sanded before it can be painted or wallpapered.
Tools And Materials
To begin the process, the necessary tools and materials must be gathered. The primary material needed for skim coating is joint compound, which comes in either a powder that must be mixed with water or in a ready-made mixture.
It is important to note that the powder version often dries faster than the ready-made one. Other tools needed include a mud pan for mixing the compound, a taping knife or trowel for applying it to the walls, a roller brush (if the compound is thin), a blister brush or paintbrush for smoothing dried joint compound, and a square carrier called a hawk (used as a palette to hold compound and move it around the room more easily).
Skim Coating Process
The first step in the skim coating process is to prepare the joint compound, if using the powder version. This is done by adding small amounts of water and mixing it until it becomes liquid but not watery. The consistency should resemble that of cake batter.
If the mixture is too watery and doesn’t stick to the trowel or blade, more powder should be added.
Before applying the compound, the wall or ceiling should be prepped by lightly sanding it, scraping off all loose pieces, and cleaning it with a vacuum or damp cloth to remove all dust. If there are any open joints, they should be taped.
The next step is to apply about 1/8 inch of compound to a small area of the wall. Any excess should be scraped off the knife and gently dragged across the area to smooth it. If the compound has already hardened, a blister brush or paintbrush can be used to add a little water to soften it before smoothing.
Once the first layer has dried, any remaining ridges or bubbles should be scraped off with the knife and rough areas should be lightly sanded. The area should then be cleaned again before applying a second coat, working in the opposite direction as the first. Once all layers are dry, the wall or ceiling should be sanded before wallpapering or painting.
To ensure the best results, it is important to work in sections of about 3 feet square, so there is enough time to smooth the compound before it dries. If working on an entire wall, it is best to divide it horizontally down the middle and work on the top half first, smoothing the compound down from the top, before moving on to the bottom half and smoothing up from the floor.
The knife should be cleaned regularly to prevent buildup of compound. Finally, all materials should be cleaned well immediately after use to prevent the mud from hardening and coating the tools.
When To Skim Coat
Skim coating is a process in which a thin layer of joint compound is applied to a wall or surface to create a smooth and even finish. This process can be time-consuming and may not always be necessary. However, it is particularly beneficial in areas where lighting is critical, such as in rooms with skylights or obliquely shining light sources.
It is also recommended to skim coat if you plan on painting the walls with a gloss or semigloss paint as reflective paint can exaggerate any defects in the surface. When applying a different type of texture, such as a knockdown or eggshell, a skim coat is generally not required.
Painting Over A Skim Coat
When painting over a skim coat, it is important to prime the surface before applying the topcoat. The primer contains a sizing agent, typically polyvinyl acetate, which helps to seal the porous material and provide better adhesion for the paint.
Before priming, allow the skim coat to dry and lightly sand it with a pole sander and 120-grit paper to remove any tool marks, ridges or other defects. Additionally, you can enhance the coverage of the topcoat by tinting the primer in the direction of the final color.