Instructions on packaging and in specifications will always call for surfaces to be dry before coating. Recently I saw a perfect example of the results if these instructions are not followed. A contractor had applied a water-proofing acrylic to a newly cast concrete roof, and blistering of the coating occurred in some areas within a week of application. The moisture, still present, in the underlying substrate was prevented from evaporating by the non-porous coating, and pushed the coating off the surface in water-filled blisters. On investigating the problem it was found that the moisture content of the slab was as high as 30%. Ideally, coating should only proceed when moisture levels are 12% or less.
Water-proofing contractors should carry a suitable moisture meter to ensure that the substrate is sufficiently dry before coating. Leaving a rubber mat on the surface over-night is an alternate method of detecting high levels of moisture. A dark patch will be present under the mat when lifted, if there is a high level of moisture in the substrate.
The moral of the story is to check first - rather than to make a costly error.
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