In our article "Repairing Peeling Plaster Top Coats" we give you a very brief description of how a plaster over lath wall was constructed. If you have read that article you know that we are calling the scratch and intermediate coats the substrate. Others may call them the brown coat, the mortar coat or even the sand coat. No matter what they are called the all define the sandy mortar like mix that was applied to the lath in two or more stages to build up the wall before the final white top coat was applied.
When this substrate fails the plaster may fall off the wall or ceiling leaving exposed wood lath. General causes of failure include moisture in side the walls or the fingers that hold it tight break off. Home that have been subject to excess temperature swings such as one experiences when there is no heat in the winter may also experience various forms of plaster failure.
Whatever the cause the cure is the same; you need to remove all the loose stuff in order to do the repair. This is a good time to mention that a quality dust mask or respirator should be worn during the removal process. You not only need to remove the failing plaster you need to dislodge and remove as much of the broken fingers that are trapped between the lath as you possibly can while not making matters worse. Old flat blade screwdrivers or a painter's 5-n-1 tool are good choices for this.
There are several different methods of repair depending on the size of the damaged area. The very first step in all of them is to clean the area completely after the failing plaster has been removed. Using a stiff broom or brush sweep down the lath to remove as many crumbs of sand as possible. A shop vac with a high quality filter also works well.
The size of the failure will determine which method you choose. There are no established guidelines. The sizes mentioned below are my preference based on my years of experience with the stated goals of doing the best I can as quickly as I can and not always for the lowest cost.
For a 2x2 foot area most likely I would use a setting type non-sanding drywall compound such as DURABOND 20 applied directly to the lath and built up to almost level with the wall surfaces. Before the first application I would mist the old lath with plain tap water from a spay bottle. This settles the dust and helps prevent the super dry wood from pulling the moisture out of the compound too quickly; dampened, not dripping!
Most likely this will take two coats to fill the void. One bag of compound should do the job. Then for the final coats I would either use a setting, sanding type compound or just plain old pre-mixed compound. The DURABOND type of product is very similar to the original plaster so I generally do not tape the area where it meets the plaster.
For larger areas I usually use either 3/8 or 1/2 drywall, whichever is the correct thickness, screwed to the lath and studs or joists. If the lath is still well secured there is no need to hit a stud at every location you screw. I use the setting type non-sanding drywall compound to fill in the irregular edges as it is very seldom that you can cut a perfect square or rectangle in the old plaster. Where the drywall meets the fill and where the fill meets the existing surface I bed tape and finish like it was regular drywall. This process is discussed in a little more detail in our article, "Replacing Plaster with drywall".
Another option is to fill either void the way the pros did by making your own scratch and intermediate coats. A few bags of Portland cement based mortar mix with some added line to make it stickier will work nicely for this; it can be extended a little with the addition of some extra sand. Chances are good you won't get this in two coats the way the pros did. Mist the wall between coats. I usually reserve this method for when I have to fill over a masonry surface like a around a chimney.
