Gypsum Plasters

Gypsum plasters are often what comes to mind when the term plaster is used, since gypsum was the base for the old style lath and plaster walls common in many older homes in North America. Gypsum is also the main ingredient used in making modern drywall compound and sheets.

Durability

Use gypsum plasters only indoors, since exposure to rain will cause rapid deterioration.As an indoor plaster, it is can last the lifetime of your home with no maintenance required.

Cost and Availability

Gypsum plasters are manufactured by many large companies under different brand names — Structolite, Sheetrock 90, Durabond 90, and Red Top are common — and most building supply stores will be able to order gypsum products if they don't stock them. Gypsum plasters are fairly expensive as base coats, especially if they are used directly from the bag without the addition of any sand or other aggregate. Prices tend to vary several dollars per bag for the same products, so check several sources before you buy.

Mixing and Application

Gypsum plasters come ready to mix with water. This must be done in relatively small batches, since gypsum plasters set chemically (except premixed joint compound), and each batch has a short workable lifetime (15 to 60 minutes). Some gypsum plasters include perlite in the mixture to add volume and reduce weight for base-coat applications. Fine screened sand or other aggregate can also be added to base coats to give extra volume; most commercially available bags include ratios for its addition. It will take some experimentation to judge what volume to make for each batch.

Gypsum plasters are easy to apply, since they are quite sticky and pliable while wet, making application on difficult spots like the underside of window openings relatively easy. Sculpting and relief work is also very easy. The gypsum adheres well to bare straw or a base coat of any other plaster, and look good if allowed to take the shape of the bales underneath. If your aim is to achieve straight walls, you will have to master the techniques of the old-fashioned plasterers, which can take a lot of practice.

Code Compliance

Gypsum plaster is acceptable as an interior finish in most areas. It has a long history in home building, and the companies that manufacture modern varieties have testing data on strength and durability.

Embodied Energy

Gypsum must be mined and heated like cement and lime, but the process requires only about one third of the energy. Manufactured products include chemical stabilizers and agents to slow or speed the setting process.

Quality of Finish

Gypsum plasters provide a soft finish and are not prone to dusting or cracking. They create a more pleasant acoustical effect than do cement/ lime finishes. Some varieties dry to quite a bright white finish without the addition of any paints or pigments. Gypsum plasters take regular paints very well, or any number of pigment solutions can provide color. It is easy to sand a gypsum plaster finish to remove high spots and achieve a smooth finish. Patching is simple, since small batches can be easily mixed, and successive layers bond well to one another.

Other Factors

You might want to purchase a single bag of several different gypsum plasters and experiment to see which gives you the qualities you most prefer. The carefully engineered mixtures and the consistency of the mixes allow for generally crack-free walls. Gypsum plasters can be used as a top coat over any base plaster, providing your interior with a softer, more pleasing finish. You must clean your tools and mixing equipment frequently, since curing occurs very quickly.

Combination Plasters

None of the four main types of plaster has to be used on its own. It is possible to combine ingredients to make hybrid plasters or to use different plasters for different coats.We have tried reducing the cement content in our cement/lime plasters to as little as 20 percent without affecting the curing time or the end results. It is also possible to make earth/lime plasters that have some desirable qualities. Although gypsum and earth can be combined, gypsum and cement will start to get rock-hard very quickly! Before you make any unorthodox combinations, it is best to do some research or try some experiments — before you are committed to covering your entire house!

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