DASH BOND COAT
A dash bond coat, sometimes called a slush coat, is a fluid mixture of cement, fine sand and water which is dashed onto the surface of a substrate to improve the adhesion of subsequent application of cement plaster to the base. The slush coat may be dashed onto concrete, clay bricks, existing cement plaster or concrete block walls. A dash bond coat is an excellent, time-proven method of improving the adhesion of Portland cement plaster to walls and soffits of buildings, slabs, the interior of swimming pools and to block walls of dairy barns, among other applications.
Attention to several important factors will assist in development of good bond to the substrate. If a dash bond coat is not applied properly, it might be preferable not to slush the wall at all.
It is important that the dash bond coat not be allowed to dry too quickly or the adhesive value of the cement paste will be lost. A slush coat that has been allowed to dry too quickly, without reasonable opportunity to cure, may be easily peeled from the back-up wall. When adhesion of the slush coat to the base is inadequate, any coat of plaster applied over that dash coat will not adhere well to the wall or ceiling. If serious drying conditions exist at the time of application of the slush coat, moistening at least one time with a fine fog spray of water is beneficial.
PREPARATION OF THE SUBSTRATE:
Be certain that the wall or ceiling is clean, free of oil or grease, and absolutely free of any type of form release compound. Splashing clean water onto several places on the wall to determine if there is quick and positive wetting of the base by the water is a good system for testing the wall for acceptance of the dash coat.
If suction in the base wall is excessive, overcome some of the absorption by spraying a LIGHT mist of water onto the wall and allow a few minutes for absorption to take place. Do not soak the wall. Dash the mix firmly onto the wall. If the wall is of high density concrete and there is no indication of retained form release compound, it is advisable to incorporate some good quality bonding agent in the mix water.
PREPARATION OF SLUSH COAT:
One volume of portland cement (plastic or regular) mixed with one to two volumes of fine sand is a suitable ratio. The aggregate should be as fine as masonry sand or finer. Plaster sand is usable but not as desirable for this purpose because of its relative coarseness. Add sufficient water, or a blend of bonding agent and water, to make a paste of such consistency that it will stick well to the wall when cast against the substrate.
Avoid dashing a wall or pool during extreme heat unless provision is made to shade the substrate against the intense hear of the sun. It is good practice to apply the dash coat after the peak heat of the afternoon has passed, or in the evening, if protection is not provided. Otherwise, a curing compound compatible with the cement plaster should be sprayed over the dash coat to retain moisture in the membrane.
If a drying wind is blowing and application of the slush coat cannot be postponed, compensate for the rapid drying action of the wind by covering or screening the wall, by moistening the slush coat with a fine mist of water or by application of a coat of compatible curing compound.
If the plasterer elects to moisten the dash coat, a fine fog spray should be used, not a stream of water.
When preparing to re-plaster a swimming pool, it is good practice to incorporate a bonding agent with the dash coat. Some of the available bonding agents should not be used in pools in the manner that they would be employed on a typical above-grade wall. The writer does not suggest applying a film of liquid bonding agent to the surface of the substrate of a pool below water level. Instead, it is good practice to inter-mix a bonding agent with the mix water that is to be blended with the cement and sand of the dash coat. Be guided in these cases by the instructions of the manufacturer of the bonding agent.
It is not advisable to use a reversible type bonding agent in swimming pools.
When re-plastering pools, a dash bond coat cannot adhere to the substrate unless the entire surface of the base has been cleaned of soil, efflorescence, body and hair oil, sun tan lotion and hand cream. Each of these agents serves as a release agent and prevents bond, if allowed to remain on the base.
The entire surface must be sand-blasted, bush-hammered or made ready in other suitable manner as described in my technical memorandum "RE-PLASTERING SWIMMING POOLS". Sand-blasting is more effective in this regard than acid-washing. Coarse sand is preferable to fine sand as the abrading agent, because it develops a rougher surface on the substrate, for improved key.