THE SCRATCH COAT
(PORTLAND CEMENT PLASTER)
The scratch coat is critically important for a Portland cement plaster wall or ceiling of top quality. This coat can have considerable effect on the serviceability of the brown and finish coats.
Start off right by applying the scratch coat as thickly as possible. In order to build up a reasonably thick layer on the wall, the mix must not be too wet. The water-cement ratio, which should be kept as low as possible, must be controlled by the man at the mixer.
A thick, strong scratch coat is less prone to damage from outside causes and forms a superior base for the second layer. Also, a good base coat that has been relieved of any potential shrinkage is to your advantage. A better wall is formed if the brown coat is not laid on to develop great thickness, but serves mainly as a leveling and smoothing layer over the scratch coat to bring the total thickness of the base coat plaster up to the required depth.
The scratch coat of Portland cement plaster should be scored in ONE direction only, whether on walls or ceilings. Cross-scoring weakens the scratch coat needlessly, and may cause unnecessary cracks in the first layer, which could transfer to the brown and finish coats. Vertical scoring of walls causes rapid run-off and loss of curing water. One should scratch walls only in the horizontal direction so that curing water which is sprayed on the wall will be retained and absorbed into the plaster. On ceilings and soffits mark across the framing members with the scratcher (that is, at right angles to the ceiling joists) to have a stronger more effective scratch coat.
Avoid scoring the plaster deeply! Shallow scratching is sufficient to provide all the required mechanical bond between the brown and scratch coat. Deep marking weakens cement plaster needlessly, serves no effective purpose, and may contribute to cracks. Cross-scratching (or scratching in two directions) aggravates the problem seriously. The bond between coats of cement plaster is developed primarily by suction in the lower coat and by chemical reaction of the components of portland cement plaster at the juncture of the two coats.
Don't tilt the scoring tool. In other words, keep the scratching tool parallel with the wall or ceiling because tilting forces the tool too deeply into the first coat in certain places and a crack could develop in that weakened line.
Limit the content of mix water to only the amount of water needed to lubricate the mix and make it spreadable. This Is critical because excess mixing water may lead to future cracks and could weaken the finished plaster.
If the scratch coat is to remain uncovered by a successive coat of plaster for several days (that is, not doubled back) it must remain moist enough to develop strength in the plaster. After satisfactory strength is obtained, additional wetting, although unneeded, will not harm cement plaster and may be beneficial. Doubling back, of course, re-supplies the scratch coat with more water for curing, and is beneficial in that respect.
Fog spraying of cement plaster has been recommended for years. The fog spray of water performs the service of hydrating or chemically setting. the cement portion of the plaster to a satisfactory degree.
The proper wetting of the wall must be dictated by the firmness of the plaster In each of the base coats. No set amount of water nor frequency of wetting can be specified. Conditions on some jobs are sufficiently favorable for coring so that additional wetting is not needed after the plaster has been applied. The amount of curing water needed will vary on each job, depending on temperature of the air and wall, the velocity of the wind and dryness of the air (note that very severe drying conditions sometimes exist in winter), the exposure (generally, southern and western exposures require more water curing) and whether an overcast is protecting the plaster from direct sunlight.
Proper water curing of cement plaster demands maintenance of moisture In the scratch coat because the first coat, by its nature, has less mass to retain water and at the same time gives up its initial water to the air quite rapidly. Fog spraying is a simple, effective way to cure plaster. The brown coat wets the scratch coat again as the second coat is applied and introduces more water for curing. This rewetting happens again when the third coat is applied.
The first coat will be a superior base IF ENOUGH WETTING OCCURS (by spray or by doubling back) to HYDRATE THE CEMENT PASTE fairly well before drying shrinkage can occur.
It is recognized that many competitively-bid houses and apartment projects are done at a price that doesn't permit adequate curing. In these cases the builder "gets what he pays for" and he may not be paying for a reasonably well-cured job.
In summary, a properly applied and cured scratch coat reduces the possibility of problems in the two top layers of plaster, and provides an excellent base for finishing the beautifully, plastered wall that brings many years of trouble- free pleasure to the owner.