Skim coating is the process of applying a very thin covering of joint compound, or mud, to smooth out rough wall and ceiling surfaces.
This can involve the application of a single coat of mud to hide minor imperfections, 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.
Setting compound is a powder that sets to a very hard finish when mixed with water, similar to concrete. 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 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 big jobs like an entire ceiling or wall.
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 easier to achieve.
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. 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 finish.
Sand slick, shiny paint surfaces with medium 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 wipe away all dust. Coat the surface and immediately surrounding area with primer-sealer before beginning.
Use a mud pan to mix 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 mix 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 wet mud onto one side of the pan, scooping it up with the knife and scraping it onto the rim. This will help to keep it under control and avoid spreading it to unwanted areas which can create a mess and make it harder to get a smooth finish.
Apply the mud, transferring it to the work surface by "cutting" a portion from the rim of the pan and spreading it like buttering a piece of toast.
Note: After powdered compound sets for a few hours it will be hard to remove from tools, etc. To avoid the extra cleanup later, scrape off the hardened 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 it down plumbing drains, it will set in the pipes and cause a clog.
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 1/8 inch of mud.
Immediately go back and "skim" the excess mud from the surface. Hold the joint knife at a 30° angle and press down hard as you pull it in one complete stroke, from one side of the area to the other without stopping. Return the collected mud to the rim of the pan. Skim off the remaining mud with parallel stokes leaving a consistently thin layer behind. Ignore small ridges between strokes as you go, you can remove them later after the compound has set.
Between strokes scrape the excess mud from the knife blade onto the rim of the pan. Keeping the knife as clean as possible like this will make the job easier and improve the results.
When the mud sets scrape off any ridges using the joint knife in upward strokes and lightly sand with 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. Vacuum up dust and wipe the whole area with a damp cloth or sponge before applying a second coat.
At this point the damage or flaw you are trying to cover will probably still be visible through the mud. To apply a second coat use the same technique of buttering and skimming used with the first, but this time remove the mud, stroking in a perpendicular direction.
For example, if your first coat was applied horizontally, stroke vertically with the second coat. This will help to begin leveling the surface by filling any ripples and other irregularities in the first coat. Allow the mud to set and dry completely.
Apply as many subsequent skim coats 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 and wipe the dust away with a damp rag before applying another coat.
Always alternate the direction of your strokes with each new coat of mud. Always clean the knife blade off on the mud pan after each stroke.
To skim a whole wall, work on a manageable swath of a few feet at a time. Working from the ceiling to about midway down the wall, apply the mud with back and forth strokes and immediately skim off the excess working from top to bottom.
Next, move down and work from the baseboard up to the midpoint. Skim the excess stroking up to overlap the top part a bit to blend the two parts together. Work in small rows like this, across the wall to the next corner.
On ceilings work across in 3 or 4 foot rows, breaking each into manageable sections. Work all along the corner with one wall, stroking back to overlap each new section and blend the coat together. When you reach the other side of the room, start a second row across the ceiling working parallel with the first. Blend the edges of the mud together along all edges as you go. If a second coat is needed, skim using strokes that are perpendicular to the first to aid in leveling the ceiling.
When you've built up the surface so it's sufficiently to smooth, allow the final coat to set and dry completely. If you're using ready-mixed compound it's best to leave it to dry overnight. Sand the finish lightly with 120 sandpaper for setting compound and 220 for ready-mixed, feathering the edges into the surrounding surface.
Sand a whole wall working from the ceiling down to the floor and across to the next corner. Sand ceilings using a sanding pole working along one wall and then out toward the other side of the room.
Vacuum or brush the surface and wipe lightly with a damp cloth or sponge to remove all the 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.