Q: What is the only base coat plaster that can be used as the scratch and brown coat for a conventional 3-coat application over wide-flange (1 inch or greater) framing?

A: RED TOP® STRUCTOBASE gypsum basecoat plaster.

Q: What is the difference between IMPERIAL^® Finish Plaster and DIAMOND^® Interior Finish Plaster?

A: IMPERIAL Finish Plaster is the stronger and more abrasion resistant. DIAMOND Interior Finish Plaster is designed for ease of use and slightly improved abrasion resistance when compared to gypsum panels.  

Q: Can RED TOP^® Brand Finish Plaster be applied over Veneer Basecoats?

A: Yes. Mill-mixed RED TOP Finish Plaster or other job site lime/gauging plasters can be applied over Veneer Basecoats.  

Q: Can plaster be used for grouting doorframes?

A: Yes, but only on open throat doorframes, per door manufactures requirements. RED TOP^® Gypsum Plaster mixed with sand (1 part Red Top Gypsum/ 2 parts sand per bag recommendation) is most commonly used for full grouts. See the Gypsum Construction Handbook for full details.  

Q: What is the best way of repairing an old wood lath plaster application?

A: Remove the old plaster down to the wood lath, staple metal over the existing lath and apply RED TOP^® sanded conventional basecoat over the metal lath. Any USG finish plaster can be applied over the basecoat plaster for a final coat.

Q: What paints are recommended for decorating veneer plaster?

A: SHEETROCK^® Brand Wall covering Primer (or equivalent) should be used to prime a veneer plaster surface. Lime containing veneer plasters (such as DIAMOND® Interior Finish) should be treated with an alkaline resistant paint (pH greater than 12).

Q: When can a Conventional Plaster System be painted?

A: A Conventional Plaster system can be painted after it is completely dry.

Q: Can conventional 3-coat plaster be used with drywall grid?

A: Yes. If the main tees, cross tees and hanger wires are properly sized and spaced to carry the plaster load and only RED TOP® STRUCTOBASE gypsum basecoat plaster is used for the scratch and brown coats.

Q: What is the shelf life of plaster?

A: Plaster can be stored 3-6 months if it is stored in a cool, dry place, protected from extreme heat and cold, direct sunlight and at a temperature above 45 °F (7 °C).

Q: Can veneer plaster be applied to sun faded IMPERIAL^® Brand Gypsum Base?

A: No. An alum spray must be applied prior to application.  

Q: Can USG Veneer Plaster be applied directly over CMU?

A: The CMU must cure for at least 45 to 60 days prior to the plaster application. Refer to USG system folder SA920 for specific application directions.

Q: What is the recommended stud spacing for installing conventional plaster?

A: Do not exceed 16 inches on center.

Q: When can veneer plaster be painted?

A: Veneer plaster can be painted when it is completely dry. The best way to determine if the plaster is dry is by taping a piece of 12" by 12" plastic to the surface. If any moisture remains on the plastic after 24 hours then the plaster is not completely dry.

Q: What is the recommended spacing of control joints for a plaster system?

A: Control joints are not to exceed 30 feet for walls and ceilings. The spacing for a ceiling with perimeter relief is not to exceed 50 feet.

Q: What are the deflection design criteria for installing plaster systems?

A: When installing a veneer plaster system it should be designed at deflection L/240 and L/360 for a Conventional Plaster system.

Q: What is this white powder on the face of the finished piece?

A: The piece was not dried correctly.

Q: Why are there large/hairline cracks on the finished pieces?

A: The pieces were probably not handled properly. Wet pieces have about half the strength of a dried piece making them vulnerable to abuse.

Q: Why is the finished piece rough and feels like sandpaper?

A: Large particles in plaster slurry settle out against the face of the mold. Mix until the plaster "creams". Slurry is then thick enough to prevent settling.

Q: Why do the finished pieces appear white and chalky even though I used a colored plaster?

A: The pieces might be calcined (burnt) because of extremely high drying temperatures. Regulate temperature of dryer at about 120ºF and remove pieces as soon as they are dry.

Q: Why are there black, green or reddish-brown spots on the surface of the finished piece?

A: The piece was not dried correctly.

Q: Why did the plaster slurry set before it was sufficiently soaked and completely mixed?

A: Determine if the plaster became contaminated after its removal from bag. First mix plaster from the storage bin in one mix. Then, under identical conditions, use plaster directly form the bag for a second mix. Any lumps of setup plaster in bags or storage bin should be removed. If both mixes set too fast and do not appear contaminated, the plaster may be at fault or either of the two following causes may be present: 1) Dirty mixing equipment or 2) Impurities in mixing water.

Q: Why was air trapped on the surface of the finished pieces?

A: The plaster was improperly soaked or mixed. Start with a[!]good quality plaster. Sift plaster into the water at one time. Let the plaster soak completely until all particles are wetted. Thoroughly mix with a mechanical mixer having the propeller adjusted so air is not whipped into the mix. Mixer should be set at a high speed (approx. 1,750 rpm). Air entrainment is a widespread problem and should be constantly guarded against. Always add plaster to water, never the reverse.

Q: Why won´t paint stick or blisters/peels off of the surface of the finished piece?

A: The plaster was not dried correctly.