Applying Plaster Properly
Plaster walls used to be common in homes, but people left installing these traditional three-coat walls to the professionals and began utilizing drywall for DIY work. Although less common today, three-coat plaster walls can still add beautiful, traditional character to your home. You don’t have to hire a pricey, skilled tradesman to get the job done. Follow these simple steps to perfect the three coats it takes to get traditional plaster walls in your home.
Step 1 – Apply the Scratch Coat
The first coat, called the “scratch coat,” is a mixture of lime or gypsum, aggregate, and water. This mixture is applied in a thick layer about 1/8- to 1/4-inch thick to a wall covered with either traditional wood or metal lath.
If you have a choice, metal laths are preferable for plaster work. Wood laths are horizontal strips of wood nailed to a vertical, upright timber framework with 1/4-inch gaps between each lath. Because moisture in wood can cause it to expand and contract, it can crack the plaster. However, metal laths don’t contain moisture and are therefore a suitable choice.
To apply the first coat, lay it on as thickly as possible in an arching motion. To do this, the water-cement ratio should be as low as possible. Catch any falling plaster by keeping the hawk under the trowel. Then, “scratch” or “score” it with a plaster scarifier, or comb, to create grooves to help the second coat bond to the coat below. While some people cross score, this can weaken the coats and cause cracks. Instead, try scoring it shallowly in a single, horizontal direction. Let this coat sit for 48 hours.
Step 2 – Apply the Brown Coat
After the first coat has dried sufficiently, the “brown coat” is applied. This is the same plaster mixture again applied in approximately the same thickness. After applying, run a slicker, or a “browning rod,” over this coat to ensure there are no holes in the plaster surface and the wall is smooth from top to bottom. Let set for another 48 hours.
Step 3 – Apply the Finish Coat
After allowing the first two coats to dry thoroughly, it's time to apply the finish coat. This coat is essentially a mixture of lime putty and water that is trowelled on in smooth arcs with the trowel never leaving the wall.
As the plaster begins to dry out, it is “water trowelled,” or sprayed with small amounts of water and trowelled to a smooth finish. Let this finish coat set for approximately one week.
Step 4 – Prime and Paint
Once a plaster wall has dried thoroughly, it is ready to be primed and painted. There is no need to sand or smooth a properly finished plaster wall; in fact, sanding will only weaken the final finish.