Skim coating is the process of applying thin layers of drywall joint compound (mud) across sections of, or the entirety of, a wall or ceiling.
Getting this right is crucial if you plan on ever looking at your walls. The reason for this is that skim coating leaves a flat surface; walls may then be painted or wallpapered. The number of coats that must be applied rises proportionally to the extent of the underlying imperfections; although, the layer of joint compound only needs to be thick enough to provide a flat surface across the wall or ceiling. Two or three coats are generally needed.
Note that if your drywall is cracked or crumbling, just skim coating will not be enough, the drywall needs replacing.
Here’s a brief guide to the process of skim coating your own wall/ ceiling:
Preparing the surface
Ideally, a skim coat should be applied directly over drywall or another joint compound; however, most paint surfaces will be fine, provided it isn’t too shiny—more time will be needed prepping such a finish want these finishes sanded first.
The surface should be prepared as if for painting: this means making it clean, using a degreaser if required; apply a water-based primer to paint that isn’t matte; if your wall has accumulated layers of paint and plaster over the years, you should consider stripping these layers away—you don’t want thirty years of paint and plaster falling to the floor upon the weight becoming too great.
Skim coating is a time-intensive process. Don’t start out unless you’re willing to commit the time to get it right. You should also be warned in advance about the amount of dust that will be produced in the sanding stage, which is considerable.
Materials and equipment
Joint compound (also called ‘mud’)—Use setting compound or ready-mixed joint compound. Ready-mixed joint compound is bought ready to apply, it will start to dry out after being opened and worked with.
The setting compound is the same powder without the water; the time periods for setting are variable, and the rate required depends on the size of the area being skim coated: for large jobs applying multiple coats, use a slow setting compound (of around an hour or more). Don’t use topcoat compound on top of the paint, it will only adhere well to a previous layer of joint compound.
Sandpaper—For setting joint compound 120; ready-mix compounds are typically softer, you should use 220 with these. If the area you want sanding is large, use a sanding pole with integrated dust collection.
A plasterer’s trowel—but don’t borrow one from a professional, we tend to use longer more unwieldy versions of the 12” trowel that would be ideal for those who haven’t done this before.
If you’re working on a high wall or ceiling, a ladder, scaffolding, or stilts
A bucket for mixing setting compound
A method of stirring the compound—e.g. a drill with a mixing rod attached
A shallower pan for holding prepared compound in
Mixing your joint compound: If using setting compound, do this in small batches; if using premixed compound, you may need to add a little water, in addition to stirring, the consistency should be easy to work with but should not run (remember you can always add more water, but you can’t remove it from the mixture).
You will need to clean out the vessels you are using to mix and hold your compound; else you will be applying lumps of the dry compound to your wall.
Apply a layer (1/8 to ¼ of an inch) of the compound to the wall. Just like butting a piece of bread, you should keep the angle of your trowel somewhat less than 45 degrees.
Start from the top of a wall working downward, or from the edges of a ceiling. Keep the edge of the trowel as clean as possible throughout to ensure the compound is applied smoothly. Load the trowel in the center, the compound will move to the edges as you apply firm uniform pressure.
Once you have covered the entire surface, allow the mud to set. Where there are large ridges, such as where each application overlaps, sand these down gently.
Corners are hard. There will be excess mud here, use a joint knife to gently remove the large part of this excess. Then use a paint scraper to get a little more. Lastly, patient use of a sanding block will yield a sharp result.
The second layer should be applied in almost the same manner as the first, the difference being that you should be skimming at 90 degrees to the way you were previously: i.e. if you were ‘buttering’ your wall from right to left, you should now be going up to down. Applying perpendicular layers will go a long way to smoothing out the ripples produced.
If, after the second coat is applied, it now looks as if sanding will now make the wall perfectly flat, you may stop. If not, apply a third coat…
Take a break. Let the mud set completely overnight (setting compound will take less time than ready-mix to set; it will be ready to sand in several hours).
Using the correct paper, sand the entire surface—removing any bumps or ridges. Whereas when applying joint compound, you will have worked in long straight lines, you now want to use large arcs.
Wear a respirator—there’s going to be dust like you won’t believe.