There are actually three layers of plaster applied to lath to make the complete job. The first two are a mortar like stuff that sometimes contained animal hair and is the origin of the term horse hair plaster. The first coat is called the scratch coat and was applied with firm pressure to force the mortar through the gaps in the lath and create the fingers that help to hold it in place.
The second coat was know as the intermediate coat and was the same mixture as the scratch coat. This intermediate coat built up and leveled the surface. Collectively these first two coats are grouped as one and called the brown coat by most people. They are grouped because there is no way you will separate those two layers once that have cured. We are going to call the brown coats substrate in these articles. If the substrate has failed you will need to go all the way to the lath to affect the repair. We will deal with that topic in another article.
One of the plaster failures that sometimes occurs is where the hard coat, also know as the top coat, the white layer pops loose from the otherwise sound layer of the substrate, the mortar like coat. This is most common on exterior walls, bathrooms and kitchens though I have seen it in other places where one would not suspect moisture being present.
In a typical installation the hard coat or top coat is normally no more than 1/8" thick and sometimes it is less. Repairing even a large section of it is no big deal if the substrate is firmly attached and sound. The size of the problem area will determine the methods required.
Products made for finishing drywall are all I ever use and they work great! Using real plaster is an art form I have never come close to mastering.
This is one of two situations where I prefer to use Durabond. The manufacturer states and based on my personal experience I agree that the Durabond product has a superior grab and hold when compared to other drywall compounds when it is applied over the plaster substrate. Since Durabond is not a sanding type product one needs absolutely sure to keep all filled areas well below the final wall finish level.
Multiple coats may be applied after each prior coat is set. Before the final coat or coats of a sanding type setting compound or even regular old premix is applied the Durabond need to be completely dry. Dry usually takes 24 hours from the last application and should not be confused with set which can occur in a little as 5 minutes.
The exact method you will pursue will be based on the overall size of your faulted area. Before we begin I strongly suggest that you pick at the edges of your fault to make sure the entire loose top coat has been removed. Once you are satisfied that nothing else is about to pop off in the near future you have to plan an attack for repairing the damaged area.
Most of these top coat pops that I have encountered that I considered worth repairing by skim coating were less than 8 square feet so I will explain the methodology for doing a 2x4 foot area and one two to three times that size. For either one the first step is exactly the same. Cover the entire substrate with the smoothest, thinnest coat of Durabond that you can apply. The substrate is always a semi-soft friable material and every time you pull a drywall knife full of compound across it you will dislodge some small crumbs or sand mix; those crumbs or sand will foul your compound and make the ultimate finish more difficult. The sooner you seal that surface and prevent that from happening the smoother the rest of the repair will go.
In either of the two following scenarios your ultimate goal is to never put any non sanding mix on the wall above the finished grade.
For a small 8 square foot or less area I suggest you skim the perimeter on two different sessions doing opposing sides first. Do the left and right and then the top and bottom after the first is set. Keep moving towards the center in both directions. Once you are close to finished grade, change over to a sanding type product and repeat the process. Some sanding is inevitable with this process. Your final goal is a wall that looks and feels smooth. Keep in mind that I said looks and feels. That is not the same thing as is.
For a much larger area I would apply a few thin stripes along the shorter dimension where it is easier to keep a thin even coat and fill the voids, always feathering to create the flattest possible surface. This is the exact same method used to skim coat an entire wall. Change over to a sanding product when you get to the point where you might need to sand to achieve the perfect smooth wall. As we said before in another article, it always easier to do another thin coat than it is to sand off a hastily applied glob.
