Applying Plaster Properly

by Murray Anderson

In days gone by, plaster walls were common in almost all homes. However, traditional plaster walls had a number of disadvantages such as the necessity for highly skilled (i.e. costly) tradesmen to apply the plaster and finish the walls properly. As well, since a properly finished wall requires three coats of plaster it took a long time to do the job from start to finish.

The development of drywall has made finishing walls a much simpler task. So easy that it’s now something DIY’ers can do (if they have some patience) while greatly reducing the time needed to finish wall. However since traditional ‘three coat’ plaster walls are still stronger and more durable than drywall and provide increased fire and noise resistance, some folks are still willing to pay the price (in both time and money) to have traditional plaster walls in their homes. Here’s how plaster walls are installed.

Traditional three-coat plastering involves two coarse or rough coats covered by a smooth third finish coat.

First coat – the scratch coat

The first coat, called the ‘scratch coat’ is a mixture of lime or gypsum, aggregate (sand) and water. This mixture is applied in a thick layer about 3/8” thick to a wall covered with either traditional wood or metal lath.
Wood laths are horizontal strips of wood (about 1wide by 1/4 inch thick) nailed to a vertical upright timber framework with 1/4 inch gaps between each lath. Since moisture in wood can cause to expand and contract and crack plaster, metal lath that doesn’t contain any moisture has become a more popular choice.
After applying the first coat it’s ‘scratched’ or scored with a ‘scarifier’ (comb) to create grooves for the second coat to help the second coat bond.

Second coat – the brown coat

After the first coat has dried sufficiently, the ‘brown’ coat is applied. This is the same plaster mixture and it again is applied in a layer about 3/8” thick. Once the wall has been ‘plastered’ a ‘browning rod’ or straight edge is run across the surface to ensure there are no holes in the plaster surface and the wall is smooth from top to bottom.
Next, although the sand in the mixture theoretically provides enough texture for the third and final coat to bond with, a cement brush may be dragged across the surface to slightly roughen it and help hold the finish coat.

Third coat – the finish coat

After allowing the first two coats to dry thoroughly the final finish coat is applied. 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 to eliminate trowel marks.
As the plaster begins to dry out it is ‘water trowelled’ (sprayed with small amounts of water and trowelled to a smooth finish).

The finished product

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.
Applying plaster properly is a job for a skilled craftsman, because even marks so up readily it isn’t normally considered a job for even a skilled DIY’er.

Murray Anderson is an experienced freelance writer over 500 articles published on the web as well as in print magazines and newspapers in both the United States and Canada. He writes on a wide range of topics and is a regular contributor to DoItYourself.com. He can be contacted at murand@lycos.com.