Building Design and Construction Handbook - Merritt - Ventech!

WALL, FLOOR, AND CEILING SYSTEMS 11.61
The brown coat not only forms the base for the finish coat, but is also the
straightening coat. The plaster should be laid on with a steel float, trued with rod
or darby, and left rough in preparation for the finish coat.
11.25.9 Finish Plaster Coats
Several types of plasters are available for the finish coat (Figs. 11.20 and 11.21).
Usually, lime is an important ingredient, because it gives plasticity and bulk to the
coat.
Gaging plasters are coarsely ground gypsum plasters, which are available in
quicksetting and slow-setting mixtures; so it is not necessary to add an accelerator
or a retarder at the site. Gaging plasters also are supplied as white gaging plaster
and a slightly darker local gaging plaster. Finish coats made with these plasters are
amply hard for ordinary usages and are the lowest-cost plaster finishes. However,
they are not intended for ornamental cornice work or run moldings, which should
be made of a finer-ground plaster. Gypsum gaging plasters should conform with
ASTM C28. Application should conform with ASTM C842.
Typical mixes consist of 3 parts lime putty to 1 part gaging plaster, by volume.
If a harder surface is desired, the gaging content may be increased up to 1 part
gaging to 2 parts lime putty.
The lime is prepared first, being slaked to a smooth putty, then formed on the
plasterer’s board into a ring with water in the center. Next, gaging plaster is gradually
sifted into the water. Then, aggregates, if required, are added. Finally, all
ingredients are thoroughly mixed and kneaded. Alternatively, materials, including
Type S hydrated lime, may be blended in a mechanical mixer.
The lime-gaging plaster should be applied in at least two coats, when the brown
coat is nearly dry. The first coat should be laid on very thin, with sufficient pressure
to be forced into the roughened surface of the base coat. After the first coat has
been allowed to draw a few minutes, a second or leveling coat, also thin, should
be applied.
The base coat draws the water from the finish coats; so the finished surface
should be moistened with a wet brush as it is being troweled. Pressure should be
exerted on the trowel to densify the surface and produce a smooth hard finish.
Finally, the surface should be dampened with the brush and clean water. It should
be allowed to stand at least 30 days before oil paints are applied.
Prepared gypsum trowel finishes also are available that require only addition
of water at the site. The resulting surface may be decorated as soon as dry. The
plaster is applied in the same manner as lime-gaging plaster, but the base coat
should be dry and, because the prepared plaster has a moderately fast set, it should
be troweled before it sets. For best results, three very thin coats should be applied
and water should be used sparingly.
Sand float finishes are similar to gypsum trowel finishes, except that these float
finishes contain a fine aggregate to yield a fine-textured surface and the final surface
is finished with a float. The base coat should be firm and uniformly damp when
the finish coat is applied. These finishes have high resistance to cracking.
Molding plaster, intended for ornamental work, is made with a finer grind than
other gaging plasters. It produces a smooth surface, free from streaks or indentations
as might be obtained with coarser-ground materials. Equal parts of lime putty and
molding plaster are recommended by the Gypsum Association for cornice moldings.