Skim coating walls and ceilings is the procedure in which an entire surface is smoothed out using drywall compound. Skim coating will produce a smooth, flawless wall. There are a number of reasons that you may need to skim-coat a wall.
I use a drywall-finishing broad knife for skim coating. Although some plasterers use a trowel, I am more comfortable using a conventional drywall knife. I use a 10-inch or 12-inch broad-knife and work out of a mud pan.
Working with chemically setting drywall compound
Chemical setting mud comes in different set (or “dry”) times. Only mix the amount of mud that you can use within the set time of the compound you are using.
Mixing quick set mud
Use a clean five gallon bucket and mix using an electric mixer and drill. Mix the compound well. Add more water if the mix is too dry. Add more compound if the mix is too loose. The mixture should be loose enough to spread easily, but not so loose that it won’t stay on the knife. Scoop the mixed compound out of the five-gallon bucket and put some in the mud pan.
Cover four by four-foot areas at a time. Starting with the top half of an eight-foot wall (for instance), apply a thin “skim” coat of compound from the ceiling down to the middle of the wall. Overlap the next strip and do the same until you have done a four-foot wide swath.
Next, do the lower half of the wall, applying a thin skim coat from the bottom up to the middle of the wall, joining the lower section to the upper section that you just completed. Repeat this until you have completed a four-foot wide swath on the lower half. Repeat the above procedure until the entire wall (or area) is skimmed out.
The Second Coat
Allow the first coat to set for the required amount of time (or longer). You may then apply the second coat, using regular all-purpose or topping compound.
All-purpose compound comes pre-mixed and dries slowly, usually taking overnight. You don’t need to worry about this compound hardening in the pan as you do with chemical setting compound.
There are two ways to do the second coat:
Allow the second coat to dry overnight (minimum), then sand. As you become skilled at skim coating, you will find that the sanding required is very light, as your walls will be pretty smooth before sanding.
I use a sanding attachment with my shop vacuum and a HEPA filter (and fine dust collection bag) for virtually dust-free sanding of the skim coat.
Vacuum the wall and remove all sanding dust, then prime with a PVA primer, and you are ready to paint with any paint of your liking.