Skim-Coat Plaster

- Continued Joint-compound tapers apply mud in straight up-and-down or side-to-side motions, whereas plasterers always trowel in arcs. I keep my wrist stiff and put my whole body into the troweling motion. Each pass is the same up-down-up motion, erasing the lagging trowel line as I go. After I reach the planned stopping point, I get on the stilts and again go left to right, arcing downward across my body. With each pass, I carefully place the trowel into the ceiling-wall corner, pulling it down to blend it into the plaster below. I feather the wetter plaster above into the somewhat stiffer, setting plaster below, taking care not to press the trowel too hard into the surface. I don't want to "pull" the plaster and disturb the set. Before reloading the hawk, I clean the back edges of my trowel -- every time.

Scratch and Double Back

The entire one-coat application is actually done in two steps, called scratch and double back (sometimes referred to as greasing). The application just described is the scratch (not to be confused with the previous step, scratching the seams). Once I'm done with the scratch, I double back over the wall in the same way, adding more plaster from the same mix, filling in with lighter trowel loads. The goal is to achieve an even 3/32-inch-thick cover of plaster over the entire surface. The scratch and double-back steps should not be confused with a true two-coat system, which I'll describe briefly later. I always plaster the larger areas first, so the mix on the table gradually stiffens as I'm scratching and doubling back. Usually, if there's any mix left after doubling back, it will be too stiff to scratch-coat any more surface. However, once the mix stiffens to the consistency of hard ice cream, it's perfect for filling in corner beads. Inside corners. I plaster inside corners in one of two ways. The first is to fill a wet corner when plastering adjacent surfaces. I'll run a corner trowel from top to bottom, or left to right, and finish with a short stroke from the opposite direction. The second method is to pull the corner on one side only, by placing a full trowel into the corner and pulling the plaster a short distance out across the surface. The adjacent side is left uncovered during that particular mix, and pulled separately later. Smoothing. After the scratch and double back, the next step is smoothing. Using a pressurized water container, I spray on a fine mist and trowel it in to smooth the plaster and fill any voids. To achieve a crisp line on a pulled corner, I run a wet, angled sash brush lightly from top to bottom just before troweling. For a wet corner, I run a clean, wet butterfly down the seam. Polishing. An hour or so after application, the plaster turns a light brown color, which indicates that it's ready for polishing. I polish with a dry trowel, pressing the trowel hard and nearly flat to the surface. Polishing gives the wall a satin-like finish. If, while I'm polishing, I notice any tiny voids, I spray the area with a light mist of water and trowel in a bit of ready-mix joint compound. It works like magic, no sanding required. During the day or two after "browning," the plaster lightens to a bright white.

Different Mix, Different Tricks

The process I just described is how I plaster using Diamond. For each plaster product, there are differences in the method: For instance, when plastering with Uni-Kal, I polish the surface by running a damp blister brush over it, followed by the trowel. This draws the finer plaster particles to the surface. Troweling drives them into the surface, producing a mirror-like finish (Figure 5).

Figure 5. After the initial set, or "browning," the plaster can be polished, using water to bring up the fines and a trowel to burnish them into the surface. It's possible to achieve a mirror-like finish with this method.

There are a few rules I set for myself, no matter what I'm plastering with. I apply the plaster mix "tight," firmly pressing it into the wall, thin and even. If it pulls with the trowel, I leave it alone so as not to refracture the plaster as it is setting. Refracturing disturbs the set, causing a longer wait before finishing. By applying plaster in thin coats, I never knock down high spots. I'm always adding, not taking away. This ensures that I never refracture the plaster. The less I run my trowel over the surface to get it right, the better the finished product, and the less fatigued I become.

Other Types of Plastering

I do a lot of resurfacing work and am often called on to repair crumbling and deteriorating plaster surfaces. Many of the homes in my area have old "horsehair" plaster over wooden lath, some more than 200 years old. If the old plaster is coming down in sheets, I remove it completely, along with the wood lath. This is the most expensive approach, because I usually have to sister in new framing, strap the ceiling joists using a laser for reference, and get everything straight before putting up the blueboard. If the old plaster is cracked but not loosened from the wood lath, I tape the cracks with self-stick fiberglass tape. Then I apply a water-based plaster-bonding agent, such as USG's Plaster Bonder, over the entire surface, and follow up with a two-coat plaster system. The plaster bonder, also called liquid lath, can be plastered over about an hour after application. I've used this process over stucco textures to make walls smooth for wallpaper. It's a good idea to apply a bonding agent to blueboard that has faded from exposure to ultraviolet light, and also before blending new veneer into existing sanded drywall or old plaster (Figure 6).

Figure 6. For a two-coat veneer over old plaster or drywall surfaces, an orange-colored acrylic bonding agent provides surface grip for the gray coat. The author also coats the facing to restore the bond on blueboard faded from UV-exposure. A 42-inch "slicker" distributes the plaster base coat evenly, eliminating irregularities.

Two-coat systems employ a base coat, or gray coat, and are useful for refinishing irregular surfaces. The base coat is applied with the same scratch and double-back steps as the veneer plaster. When applying a two-coat veneer, however, there's no need to scratch the seams to get good results. The base coat has coarse aggregates, which should not be fine-smoothed, because they provide the "teeth" to hold the topcoat veneer. There are two special plastering tools that I use to straighten a base coat. One is a magnesium darby, a 36 x 5 1/2-inch bar in the shape of a piece of beveled siding. The second is a magnesium slicker, a 42-inch-long tool similar to the darby, but a bit stiffer because it has a bent edge. I use either one to straighten the base coat once it starts to set, by scraping the tool edge over the entire surface.

Sam Singer