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In the beginning, there was plaster.
Well, that may be a bit of an overstatement, but in the United States, plaster technology has been here since the days of the pilgrims.
Materials and methods have changed and improved over the years, but some of the basics remain the same. Full coat interior plaster begins with a scratch coat (made of gypsum plaster and sand) over some kind of base. A hundred years ago it was wood lath, brick or clay tile; thirty years ago it might have been rocklath (a 3/8" sheet 16" X 48" that looks a little like drywall), metal lath, block or tile.
Today it is most likely metal lath or gypsum base (similar to drywall but with a different type of paper). The scratch coat "keys" into the base and stiffens it for the subsequent coats. It is left very rough so that the next coat, the brown coat, can get a good bond.
After curing for several days, the brown coat, consisting of gypsum plaster and sand, is applied. If thicker plaster is desired, it can be left to "set", i.e. get hard, then "double back" and put another coat of the same material right over top of it to the desired thickness. The brown coat is also left rough, although not as rough as the scratch coat.
The finish coat is then applied to a thickness between 1/16--1/8". The finish usually consists of lime mixed with something to make it set faster. The finish can be perfectly smooth, often referred to as "white coat", or textured in ways limited only by the imagination. Sand may also be added to
give a variety of effects.
Decorative elements, such as medallions, panel molding or cornice work, are often applied to the brown coat. Sticking the pieces to the brown coat ensures a better bond through "keying" to the rough coat. The finish coat is then applied around them, making a seamless, unified application.
Full coat plaster gives the most effective acoustic resistance and fire resistance. When vandals set fire to an Dayton apartment stairway recently, the owner attributes the full coat plaster with not only minimizing damage to the property, but possibly saving the lives of those asleep on the upper floor.
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