Gypsum Plaster

Hand troweled gypsum plaster is ideal for large round “Old World Santa Fe Style” corners and will also work well for a contemporary flat wall with square corners. Gypsum plaster can be built up in single and multiple coats to achieve more depth than any other plaster system available.

Traditional plaster is applied over expanded metal lath and done in 3 coats to an overall thickness of 7/8″. The first coat is called the scratch coat, the second is the brown coat and the final coat is called the finish coat. There are only a few manufacturers of gypsum plaster in the U.S. The product consists of a powder that is similar to cement or lime and is packaged in paper sacks. The base coat material will usually contain a lightweight aggregate called perlite which is mixed with plaster sand and water. The finish coat is traditionally a finer gradation of gypsum, blended only with water. When applying traditional gypsum plaster it is important to control the environment by sealing up the room that the material is being applied in. Gypsum is very susceptible to shrinking, cracking and weakening if the temperature and humidity are not controlled. Direct sunlight and wind can cause gypsum to dry to rapidly. This will cause improper hydration which in turn creates a weak product and different suction rates, affecting the final color. It is important to keep the gypsum damp throughout the process up until the the finish coat. The finish coat is usually white and painted, however, it can be tinted using approved pigment to create integral color.

Some gypsum plaster systems can be applied directly to drywall with the use of bonding agents. This type of plaster, known as “veneer plaster”, is typically applied in one thin coat. Gypsum veneer plaster is fast and usually less expensive than traditional gypsum plaster. When applying a veneer plaster, all joints in the drywall must be prepared with joint compound or with the manufacturer’s recommended joint treatment materials. Once the joints are treated, a bonding agent is required. This is a water based PVA (polyvinyl acetate) or an acrylic bonding agent. PVA bonding agents are for interior use only because they re-emulsify when they get wet. PVA bonding agents are the most popular because they can be dry before the plaster coat is applied (as opposed to an acrylic bonding agent, which must still be wet when the plaster is applied, forcing the applicator to rush the process). A controlled environment is also important when applying a gypsum veneer finish, although some veneers are modified with acrylics, cement and chemicals, providing a better bond, higher compressive and tensile strengths, crack resistance, color retention and crack resistance. These modifications may make the product easier to work with and will give the applicator more flexibility. Once the finish coat is dry and completed, it should be sealed with a water based sealer.

Hand troweled gypsum plaster is ideal for large round “Old World Santa Fe Style” corners and will also work well for a contemporary flat wall with square corners. Gypsum plaster can be built up in single and multiple coats to achieve more depth than any other plaster system available.

Traditional plaster is applied over expanded metal lath and done in 3 coats to an overall thickness of 7/8″. The first coat is called the scratch coat, the second is the brown coat and the final coat is called the finish coat. There are only a few manufacturers of gypsum plaster in the U.S. The product consists of a powder that is similar to cement or lime and is packaged in paper sacks. The base coat material will usually contain a lightweight aggregate called perlite which is mixed with plaster sand and water. The finish coat is traditionally a finer gradation of gypsum, blended only with water. When applying traditional gypsum plaster it is important to control the environment by sealing up the room that the material is being applied in. Gypsum is very susceptible to shrinking, cracking and weakening if the temperature and humidity are not controlled. Direct sunlight and wind can cause gypsum to dry to rapidly. This will cause improper hydration which in turn creates a weak product and different suction rates, affecting the final color. It is important to keep the gypsum damp throughout the process up until the the finish coat. The finish coat is usually white and painted, however, it can be tinted using approved pigment to create integral color.

Some gypsum plaster systems can be applied directly to drywall with the use of bonding agents. This type of plaster, known as “veneer plaster”, is typically applied in one thin coat. Gypsum veneer plaster is fast and usually less expensive than traditional gypsum plaster. When applying a veneer plaster, all joints in the drywall must be prepared with joint compound or with the manufacturer’s recommended joint treatment materials. Once the joints are treated, a bonding agent is required. This is a water based PVA (polyvinyl acetate) or an acrylic bonding agent. PVA bonding agents are for interior use only because they re-emulsify when they get wet. PVA bonding agents are the most popular because they can be dry before the plaster coat is applied (as opposed to an acrylic bonding agent, which must still be wet when the plaster is applied, forcing the applicator to rush the process). A controlled environment is also important when applying a gypsum veneer finish, although some veneers are modified with acrylics, cement and chemicals, providing a better bond, higher compressive and tensile strengths, crack resistance, color retention and crack resistance. These modifications may make the product easier to work with and will give the applicator more flexibility. Once the finish coat is dry and completed, it should be sealed with a water based sealer.

Author: Timothy Williams, Owner, Volterra Architectural Products

Volterra Architectural Products manufactures distinctive architectural elements for interior and exterior use including faux wood beams, corbels, brackets, shutters and truss tails. Interior/exterior finishes include limestone plaster, gypsum plaster, acrylic plaster and exterior stucco (including EIFS). Volterra also produces decorative cast resin products for the tile and stone industry, as well as manufactured stone veneer
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