What Is Skim Coating?
Skim coating is the process of applying a layer of muddy compound to smooth out rough or damaged ceilings and walls. The compound can be used to repair entire walls or fill deep scratches, holes, dents, and water-damaged portions of drywall. After applying a slab of skim coat to the damaged or textured area, the excess is skimmed off and a second layer is often added. The wall can then be painted or wallpapered without the appearance of dings, paint bubbles, or roughness.
Skim coating typically refers to the use of joint compound, more commonly called drywall compound or mud. This material is a thick but spreadable substance that dries and hardens over time, maintaining the drywall or plaster’s integrity and continuity. Plaster is a relatively soft material that is not ideal for supporting the weight of a building, but is often used to make interior walls. Plaster has been largely replaced by drywall in recent years, a material made of slabs of plaster that are sandwiched between sheets of paper and layered on top of one another.
Joint compound is often confused with the much thicker Spackle®, which is used to fill very small holes in the wall, such nail holes. Because of its thick consistency, Spackle® cannot be spread over large portions of the wall like mud. Mud can come in a setting-type joint compound or ready-mixed. The setting compound is a powder that is mixed with water to form a mud thick enough that it does not slide off a knife, but creamy enough that it can be spread. The ready-mixed compound is a pre-mixed mud that comes ready for use.
The setting mud dries at varying rates, but tends to dry much faster than the ready-mixed mud. It is important to weigh which skim coat material is appropriate for the job at hand; depending on how long the mud needs to stay wet. When skim coating a larger portion of the wall, it is better to use a slower setting drywall compound.
Once the correct skim coat is selected, it is usually placed and mixed, when applicable, in a dish called the mud pan. If a setting compound is mixed too watery, so that it does not stick to the trowel or skim coat knife, more powder should be added. A skim coat knife is a broad blade with flat edges.
Before skim coating, the wall must be prepared by lightly sanding or scraping all loose pieces off the wall and cleaning it with a vacuum or damp cloth so that all dust has been removed. The mud can then be slathered over the desired area so it is at least an eighth of inch (0.3cm) thick. The excess should be skimmed off and put back in the mud pan before the compound begins to set. After the first layer has dried, any ridges or bubbles should be scraped off, sanded gently, and cleaned. More mud can then be applied, following the same procedure and alternating the direction of the skimming for each layer.
When skim coating an entire wall, the mud should be applied in layers that extend three feet (0.9 meters) wide and halfway down the wall from the top, or halfway up the wall from the bottom. As the wall is covered, new applications of mud should overlap with the adjoining applications. The knife should be kept fairly clean throughout the skim coating. All materials should be cleaned well mmediately following the job to prevent mud from hardening and coating the tools.
Written by Caitlin Kenney
