DIY Skim Coating Techniques
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Skim coating is the process of applying a very thin coat of joint compound or mud, to smooth out rough drywall or plaster surfaces on walls and ceilings.
This can involve the application of just one coat of mud or it can involve the use of several coats, to build up a smooth, finished surface. In addition, skim coating can be done over a small area or it can be done over an entire wall or ceiling.
About Skim Coating Compounds
There are two types of joint compound that can be used for skim coating: setting–type joint compound, pictured here 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 bucket. It takes several hours to dry and is soft compared to setting compound.
Which compound to choose depends on the surface to be skimmed. For most major wall damage use setting compound. This would include any water damaged plaster or drywall, and very rough surfaces like drywall textures that will require several coats of mud to smooth out. Use a relatively slow setting compound like Durabond90® for large skimming jobs on walls and ceilings to give yourself enough time to work before it sets.
Use ready–mixed for minor wall imperfections requiring one or two coats and for finishing drywall. Ready–mixed is also good for the final coat on most skim coating jobs. It can be easier to spread and will allow for a thinner application than setting–type. It will also be easier to sand, making a smooth finish easy to achieve.
Wall Preparation
Skim coating can be done over most paint finishes with no special preparation. As long as the surface is ready for painting, it will also be ready for skim coating with joint compound. If however, a surface has been wet or you have a very shiny paint finish, some preparation will be necessary to insure no bubbling develops in the paint finish.
Sand slick, shiny paint surfaces with 120 sandpaper or prime with a flat paint to insure a good bond with the joint compound. Also repair any water damaged surfaces before skim coating. If the paint is peeling or plaster and joint compound are popping off, remove all the loose material and coat the surface with primer–sealer before skim coating. See repairing water damaged plaster or repairing drywall water damage for more on water damage repair.
Mixing Joint Compound
Use a mud pan to mix about a 2 to 1 ratio of powdered setting compound to water. Start with about a cup of powder and pour it into the pan. Add a little less than a half cup of clean, cool water. 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.
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 surface.
Note: After this mud sets for a few hours it will be hard to remove from tools, etc. To avoid the extra clean–up later, scrape off set mud right away. Use a razor–blade glass scraper or putty knife to chip the set mud from the pan and joint knife into a trash container. Don't wash setting compound down plumbing drains, it will set in the pipes and cause a clog.
Apply a First Skim Coat of Joint Compound
Apply a thick coat of mud over the surface to be skimmed in random back and forth strokes. Hold the knife at an angle to "butter" the surface with at least an eighth inch thick coat of mud.
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 or from top to bottom, without stopping.
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 with an upward stroke of the blade.
Apply a Second Skim Coat of Joint Compound
On small areas scrape off any ridges of dry mud and lightly sand with 120 medium grit sandpaper. If there are a lot of ripples, bumps or other protrusions on large areas, use a sanding block or pole sander to go over it lightly with 120 sandpaper. Vacuum up dust and wipe the whole area with a damp cloth or sponge before applying the second skim coat.
At this point the damage or flaw you are trying to cover will probably still be visible thru the mud. To apply a second skim coat use the same technique of buttering and skimming used with the first, but this time skim the mud in the opposite direction.
For example, if your first coat was skimmed horizontally, skim the second with vertical strokes. This will help to level the surface by filling any ripples and other irregularities in the first coat. 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. As you work, if there is excessive rippling or other surface imperfections, sand them lightly before applying another skim coat.
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.
Skim a Whole Wall or Ceiling
To skim a whole wall, work about a 3 foot swath from the ceiling to about midway down the wall. Butter the wall and then immediately skim off the excess working from top to bottom. When you finish with the top half, skim from the baseboard up to the midpoint, overlapping a few inches to blend the top and bottom of the wall together. Work in rows like this, across the wall to the next corner.
On ceilings work across in 3 foot rows breaking each into manageable sections. Work all along one wall, overlap each section a little to blend them together, until you reach the other corner. Continue toward the center of the ceiling in this way, skimming in rows of about 3 feet until reaching the other side of the room. Skim each row stroking all in the same direction.
Allow the mud to set for about 30 minutes. When using ready–mixed joint compound, allow an hour or more for drying.
Sand the Final Skim Coat
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 with 120 or 220 sandpaper and feather the edges into the surrounding surface.
Sand a whole wall working from the ceiling down to the floor in narrow sections, across to the next corner. Sand ceilings using a sanding pole working along one wall and then across to the other side.
Vacuum or brush dust from the wall or ceiling and wipe lightly with a damp cloth or sponge to remove all dust from the surface 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.
