Whether used as part of a decorative render or to create a luxurious marble effect, polished plaster has become an essential part of many major redecorating projects and new builds, but to get the best out of the material, you must know how and why it works so well.
Decorative plaster stucco is one of the world’s oldest architectural materials and for the most part consists of slaked lime, water and an aggregate material, which in the case of Venetian plaster is marble dust or chips.
Because of its age, it also typically needs to be applied in a specific way compared to other decorative coatings, with three specific coats and potentially up to a month of preparation to complete properly, not counting preparation.
The reason for this is that to ensure long-lasting results, lime plaster is applied using thin layers that are allowed to set separately, building up to create an impeccable result.
Rendering Coat
Also known as the scratch coat because of its use of coarse aggregate that creates a rough but optimal surface for the next two coats, the rendering coat is evenly applied to a thickness of roughly 10mm directly to the substrate before it is scored or scratched with a comb to enhance its rough texture.
The combination of its coarse materials and the scoring work should ensure an even, abrasive surface that is perfect for the other two plaster layers to be applied to. It is left to dry for quite some time, and for some finishes can take over ten days to set.
Floating Coat
Sometimes known simply as the brown coat, the middle floating coat is similarly made from a coarse aggregate and is also 10mm thick on top of the initial rendering coat.
The difference is that it is not as strongly scratched as the scratch coat, ensuring a rough
enough surface to stick but also an even and smooth enough surface that it will not affect the final product. Because the brown coat is as thick as the scratch coat, it will take a similar time to
properly set.
Finishing Coat
Also known as the skim coat, the final coat of plaster is a mere 4mm thick, far thinner than the previous two, and features a far smoother aggregate to create the optimal finish, particularly once it has been polished to the smooth sheen you would expect.
It takes four days to set, after which it can be polished, sanded, wiped and waxed to create the stunning results that so many people expect and desire.
Because lime plaster has been used since time immemorial, there have been many advances to this traditional system, such as systems that use just two coats or rapidly cut down on the setting time. One-coat lime plaster is only recommended for very rough and preliminary work.
However, traditional plastering techniques have endured for as long as they have because they are effective, long-lasting and have jaw-dropping results that speak for themselves.
Whether marble-like Venetian or metallic finishes, plaster is still mostly applied in this traditional way.
