Terminology

• Scratch coat: first coat over lath or other substrate, scored to key the next coat
• Brown coat: second coat in three-coat work
• Finish coat: last coat, a thin veneer that gives the final surface
• Base coat: scratch and brown coats together, or the coat beneath the finish
• Three-coat work: scratch, brown, and finish with set or drying time between coats
• Two-coat work: scratch and brown from one mix with no set time, then finish
• Veneer plaster: thin base coat and finish applied over gypsum base
• Gypsum base: gypsum board made as a base for veneer plaster
• Lath: metal or gypsum backing that holds conventional plaster
• Lime putty: slaked lime used in finish coats

Standards

Systems

• Conventional three-coat: scratch, brown, and finish over metal lath; the finish is typically a thin veneer; applies to lime, gypsum, and cement plasters
• Conventional two-coat: base coat plus finish over gypsum lath
• Veneer plaster: basecoat of 1/16 to 3/32 in. and a finish over gypsum base; one-coat systems apply a 1/16 to 3/32 in. finish directly to the base
• Total thickness over metal lath: about 5/8 in. for a gypsum base coat plus finish on vertical surfaces

Test and application standards

• ASTM C842: application of interior gypsum plaster
• ASTM C843: application of gypsum veneer plaster, which cites C587 for the plaster product
• ASTM C926: application of portland cement-based plaster
• ASTM C1063: lathing and furring for plaster

Materials

• Gypsum plaster: most common modern interior base and finish
• Lime plaster: lime putty with aggregate for traditional and historic work
• Cement plaster: portland cement-based coats applied per ASTM C926
• Gypsum and metal lath, gypsum base: substrates that hold the plaster

Detailing

• Match the system to the substrate; use gypsum base for veneer plaster
• Leave the basecoat surface rough and open so the finish coat bonds
• Provide corner beads and casing beads for edge protection
• Place control joints at framing movement joints and large panels
• Keep the substrate dry and allow coats to set before the next

Uses

Specific uses

• Smooth or sand-finish walls and ceilings in houses and institutions
• Historic restoration and repair
• Curved and sculpted surfaces
• Areas needing hard, impact-resistant walls

Pros and cons

Pros

• Hard, continuous, seamless surface
• Conventional systems give greater fire and wear resistance
• Veneer systems cost less and go on faster

Cons

• Conventional systems cost more and take longer than veneer systems
• Cracks at movement joints without control joints

Manufacturing and cost

Construction

• Plaster is mixed on site from bagged material, applied in coats by trowel, and floated or troweled to finish
• Veneer plaster is applied in a thin coat over gypsum base and troweled smooth

Main manufacturers and suppliers

• USG
• National Gypsum

Trade association: Gypsum Association.

Sources

• Walls & Ceilings: interior plastering vocabulary, https://www.wconline.com/articles/90661-interior-plastering-vocabulary
• USG: Imperial veneer basecoat plaster brochure, https://www.usg.com/content/dam/USG_Marketing_Communications/canada/product_promotional_materials/finished_assets/imperial-veneer-basecoat-plaster-plasters-brochure-en-can.pdf
• USG: plaster system brochure, https://www.usg.com/content/dam/USG_Marketing_Communications/canada/product_promotional_materials/finished_assets/plaster-system-brochure-en-sa920-can.pdf
• General Services Administration: applying one- and two-coat veneer plaster over a gypsum base, https://gsa.gov/real-estate/historic-preservation/historic-preservation-policy-tools/preservation-tools-resources/technical-procedures/applying-one-and-twocoat-veneer-plaster-over-a-gypsum-base
• ASTM International: C842 standard specification for application of interior gypsum plaster, https://standards.iteh.ai/catalog/standards/astm/036c5829-ab9a-4f6e-a972-892196373894/astm-c842-05-2015