Skim Coating Techniques for Do it Yourselfers

Skim coating is the process of applying a very thin coat of joint compound to smooth out a rough wall or ceiling surface. This can involve the application of just one coat of mud or it can involve the use of several coats to buildup a smooth, finished surface. Skim coating can be done over a small area or it can be done over an entire wall.

There are two types of joint compound that can be used for skim coating: setting–type joint compound and ready–mixed joint compound. Setting compound is a powder that sets to a very hard finish when mixed with water. It comes in setting rates of a few minutes to over an hour. Ready–mixed joint compound comes wet and ready to use out of the container. It takes several hours to dry and is soft compared to setting compound.

Which compound to choose depends on the surface to be skimmed. Use ready–mixed for minor wall imperfections requiring one or two coats and for finishing drywall. For most other situations use setting compound. This would include any water damaged plaster or drywall, and very rough or gouged surfaces that will require several coats of mud to smooth out. Use a relatively slow setting compound like Durabond90® for large skimming jobs to give yourself enough time to work before it sets.

Mixing Setting Joint Compound

Add some joint compound to a mud pan. Mix setting compound at about a 2 to 1 ratio of powder to water. Pour about a cup of powder into the pan, add a little less than a half cup of clean tap water and mix. Mix the mud so it's thick enough to remain on a joint knife when held upside down. If your mud is too thin and soupy, add a little more powder until you get the right consistency.

This type of joint compound will set quickly so use it right away. After this mud sets for a few hours it will be hard to remove from tools, etc. Scrape off set mud right away to avoid the extra clean–up later. Use a putty knife to scrape the set mud into a trash container. Don't wash setting compound down plumbing drains, it will set in the pipes and cause a clog.

Handling Joint Compound

Collect the mixed mud onto the rim of the pan. This will help to keep it under control and avoid spreading it to unwanted areas.

Transfer the mud to the wall or ceiling by cutting a portion from the rim of the pan and applying it to the wall.

Skimming Joint Compound

Apply a thick coat of mud over the surface in random back and forth strokes. Hold the knife at about a 30° angle to "butter" the surface with at least an eighth inch thick coat of mud. For large wall areas work from ceiling to floor applying the mud in a narrow row.

Immediately "skim" the excess mud from the surface and return it to the rim of the pan. Hold the joint knife at a 30° angle and press down hard as you skim in parallel strokes, from one side of the area to the other, without stopping. To skim a whole wall work from the ceiling to about midway down the wall. Finish out the top half of the wall and then skim from the baseboard up to the midpoint to blend the top and bottom. Work in rows like this across the wall to cover the whole area.

Scrape the excess mud from the knife blade onto the rim of the pan after each stroke. Keeping the blade of the knife as clean as possible will make the job easier and improve the results. Ignore small ridges of mud between strokes. When the mud has set use the joint knife to scrape them off.

Allow the mud to set for about 30 minutes. When using ready–mixed joint compound, allow an hour or more for drying. Scrape off any ridges and lightly sand the dry mud with 120 medium grit sandpaper.

Second Coat

At this point the damage or flaw you are trying to cover will still be visible thru the mud. Apply a second skim coat using the same technique of buttering and skimming used with the first but skim the mud in the opposite direction. If your first coat was skimmed horizontally, skim the second with vertical strokes. This will help to level the wall by filling any ripples and other irregularities in the first coat. Apply a second skim coat to a whole wall working along the ceiling, buttering and skimming a narrow row across the wall to the next corner. Work down the wall in narrow rows until you reach the baseboard.

Allow the mud to set for about 30 minutes, an hour or more for ready–mixed. Apply as many subsequent coats of mud as needed to build a smooth finish over the repair area. Always alternate the direction of your skimming strokes with each new coat of mud. Always clean the knife blade off on the mud pan after each stroke. Remember to skim the edges of the repair as you go so that the mud doesn't build up there and require a lot of sanding when you're done.

Allow the final coat or mud to set and dry for at least an hour, a couple for ready–mixed. Sand the final coat of mud lightly to feather the edges into the surrounding surface. Sand an entire wall working from the ceiling down to the floor and across to the next corner.

Brush any dust from the wall or ceiling before priming and painting. If you are using flat latex finish paint, it will be self–priming. If you are using semi–gloss or other shiny paint, prime with flat latex paint or a latex primer first.