Plaster has been around for centuries and the desire for smooth monolithic walls is still the preferred look. We know that 5,000 years ago the Egyptians burnt gypsum and then crushed it into a powder then mixed it with water to be used as a joining compound for the massive blocks to build the pyramids.
Traditionally in the US, gypsum plaster was applied in three coats. The first coat was called the "scratch coat" which was applied over wood lath or metal lath and put on at a thickness of 1/4" - 1/2" in one pass, then scratched with a scarfier or some type of rake to help "key in" the next coat. The second coat or "brown coat" was applied the next day or within a few days and long straight edges or "darbies" were used to flatten the wall. This coat was finished smoother but left a little rough to allow the final coat to bond. This second pass was also applied at a similar thickness as the first coat. The third and final coat was the "finish coat". This last coat of plaster was a much thinner application and usually hard troweled to a smooth finish. The finish coat was usually painted but before paint was available, the product was integrally colored or left the natural color of the raw materials. This look is what most will call "Venetian Plaster", "Faux Plaster" or "Polished Plaster".
Most people today will call any smooth finished wall with color variation "Venetian Plaster". Venetian plaster is more of a style or technique of multiple thin coats layered one on top of the other to create the appearance of depth. Most plaster today is applied directly over prepared drywall and is done in much thinner coats. The thickness of the plaster system will depend on the preparation of the drywall substrate. Very thin plaster systems will require the drywall to be finished to a level 4 or 5, or will require a thicker plaster base coat to cover and smooth out the imperfections in the drywall. Thicker plaster systems may not require any taping of board joints and inside or outside corners, but will require some sort of treatment before applying a finish coat.
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