Scratch Coat

ASTM C 926-98 in Section 7 Application defines the procedure for applying a scratch coat of Portland Cement Plaster. 7.2.1 The first (scratch) coat shall be applied with sufficient material and pressure to form full keys through, and to embed the metal base, and with sufficient thickness of material over the metal to allow for scoring the surface.

The scratch coat is the primary structural lamina of the system. A good scratch coat creates a good environment for a good leveling (brown) coat, just as a good lath job allows for an even scratch coat. Bedding the wire is the process of shoving the plaster under and over the wire. Troweling or darbying the scratch beds the wire when the trowel stroke is vertical. Horizontal stroking flattens the plaster. Pressure applied during the stroke forces the plaster under the wire. Less pressure on the horizontal stroke smooths the plaster over the wire and allows for the scarifying or scoring the plaster. Ie. Preparing the scratch for the brown coat. If the lath is too tightly attached to a solid substrate no amount of pressure will force the wire out and allow for embedment.

Jobsite conditions notwithstanding, an occasional bit of lath not bedded normally means that the plaster was too wet but is not cause for concern. The lath will be bedded during the brown coat. The best jobs show no wire and have an even horizontal scoring. This is done for two reasons. The scoring allows a solid mechanical bond between the scratch and the brown coat, and also keeps moisture in the scratch during the moist curing process.





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