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Skim-Coat Plaster

- Continued I use a cage mixer, which has a shearing action, to make my mixes more consistent. I purchased a masonry hammer drill to get the slow 900- to 1,000- rpm mixing speed specified on the bag. For small mixes (less than one bag), I use a plastic five-gallon bucket. For mixes of up to two bags, I use a recycled metal lube-oil can. I get the cans free at quick-lube garages and wipe out the residue. When working with other plasterers, I mix larger batches in 55-gallon containers. The wet plaster is heavy enough by itself, so I use a lightweight magnesium hawk. When working on stilts or staging, a helper, called a tender, will fill my hawk with plaster using a scoop made from a capped laundry detergent bottle with the bottom cut off. Blister brushes -- wads of felt on a wooden handle -- are effective for removing blisters, or bumps, in the surface caused by overworking the plaster. A blister brush can also help get me out of trouble if a mix sets prematurely on the surface before I have a chance to smooth it. The wet blister brush softens the surface just enough to permit smoothing.


I make tool cleaning easy by spraying a coat of WD-40 on the backside of my trowel, hawk, drill, mixer, and stilts before use. This keeps the plaster from sticking. A few quick swipes with a wet mason's brush gets the tools clean, and any plaster left to set on a tool can be dislodged easily later on. Good light. Shining a bright halogen light on my plasterwork allows me to pick up imperfections that are easily missed under dim light. Years ago, I stayed past dusk to plaster one more wall under the low light of a couple of incandescent lamps. When I returned the following morning, the sun revealed small voids that weren't visible under low light. Now, whenever possible, I work while the sun is directly on the surface being plastered. Even at 1,000 watts, artificial light is no match for sunlight, so on an overcast day, I use my halogens and pay close attention to the surface.

Sizing Up the Job

At 50 to 80 pounds per bag dry weight, plaster is heavy stuff. I stage bags by room to avoid having to move them around as the work progresses. Coverage rates are published on the bag, but I've developed my own "feel" for area requirements. There's a maximum window of 30 minutes to apply the plaster and about 45 minutes to smooth it, so I plan the order of surfaces to cover and the size of each mix accordingly. Anything requiring extra time and attention must be considered: the number of inside and outside corners, obstacles like electrical wires that are hanging out for sconce lights, plumbing fittings, and places that are too small to get at with a standard trowel. Surface prep. On plaster resurfacing jobs, when the wood trim is left in place, I tape the existing casings and baseboards. To protect the floors, I cover them with tarps or red rosin paper and roll paper over the egress paths to the cleaning station and bathroom facilities. In new construction, to make cleanup easy, I cover the subfloors and protect window and door jambs with masking tape. For reinforcement, outside wall and soffit corners receive an expanded metal mesh bead attached with 5-penny galvanized box nails every 8 inches. I tape all of the seams and inside corners with self-sticking fiberglass mesh tape. Before I mix any plaster, I run my trowel over the wall to detect any protruding screws or nails, and then sink or remove them. I also make sure that electrical wires are tucked into their boxes. If the box itself protrudes from the wall, it slows me down considerably, because I can only trowel up to it, not over it.

Getting the Right Mix

Nothing is worse than plastering with a bad mix. For consistently good mixes, I purchase my plaster from suppliers who rotate their stock often. In one lumberyard, I noticed the same pallet of plaster collecting dust for a couple of years. I wouldn't get near that stuff -- its shelf life had long expired and the chemical properties would have changed to the point that mixes would not set properly. Before mixing, I make sure that the mixer, drill, and mixing container are free from dried plaster. If hardened chunks, known as "hitchhikers," dislodge into the mix, I'll have to pick each one off the wall with my finger.


Figure 3. Plaster is highly alkaline, so it's a good idea to wear dust protection during mixing, the dustiest part of the job. Use cool, clean water for a predictable mix, avoiding temperature extremes, which affect setting time. A recycled lube container controls the splatter of a two-bag mixing operation.

Mixing. To mix a batch of plaster, I put cool, fresh water in the mixing container, then my tender quickly pours in the plaster with the mixer spinning (Figure 3). Stay away from extreme water temperatures, because icy cold water will cause fast setting, while hot water will cause the set to take much too long (the opposite of what you might expect). Manufacturers never give exact water-to-plaster ratios; for example, USG recommends 12 to 15 quarts of water per 50-pound bag of Diamond. Like a good chef, I've developed a feel for how much water and plaster to mix, so I never actually measure. I try to achieve a consistency that is something like soft-serve ice cream. To eliminate lumps, I mix Diamond for the manufacturer's maximum recommended time of five minutes. While mixing, I add water or plaster to get the consistency I like -- just thick enough not to slide off my trowel when I hold it sideways. When I plaster over Gold Bond's Kal-Kore blueboard, I use a looser mix, because the board has stronger "suction" than USG's Imperial board. I've come to prefer the mixed-brand combination of Diamond plaster over Kal-Kore blueboard.

Throwing on Plaster

First, I scratch the seams. The term "scratch" refers to a coat of plaster that is covered over by a finish coat. To scratch the taped seams, I apply a thin, tight coat of plaster and then fill in the outside metal corner beads. The scratched seams can be plastered with the very next mix. Seams that are scratched one day and plastered the next should be wetted just before plastering to keep the scratch from sucking the moisture out of the new plaster. I'm careful not to get the blueboard wet, because plaster doesn't stick very well to wet blueboard. Troweling the field. After pouring the plaster onto the mixing table, I use my trowel to corral the plaster from the table onto my hawk, and move it to the center to keep it balanced. Using the trowel, I scoop the plaster off the hawk, tilting it slightly towards the trowel. I'm right-handed, so I start just above the floor at the left-most corner of the wall, spreading the plaster up and arcing to the right across my body, as far as I can comfortably reach. Then, without letting the trowel leave the wall, I press the plaster backward in a downward sweep with the opposite edge of the trowel. I finish with a light upsweep to erase the lagging trowel line (Figure 4).


Figure 4. After scratching the panel seams, the author applies plaster in arcs, sweeping up to spread, down to embed, and back up to erase the trowel mark. The trowel never leaves the surface during application.