Water-based masonry primers

Water-based primers have been gaining popularity for some time now- not only for environmental concerns, but also as a cost consideration. The cost consideration is unfortunate, but a real issue in the market we service. One disadvantage that a water-based primer has compared to a solvent-based masonry primer is the limited penetration into the substrate. Water-based binders are emulsified, and the droplets have a measurable particle size, meaning that penetration is not possible into interstitial spaces smaller than the droplet size. As long as the substrate is well bound,dry and not friable, water-based primers perform well. An experienced contractor will be able to make the call as to whether the substrate is suitable to be primed with a water-based primer or not, whereas decisions that are solely price driven can result in unnecessary failures. Recently I saw an instance where a still wet plaster-skim had been primed with a water-based primer and over-coated with a washable sheen acrylic, with predictably disastrous consequences. The entire coating system could be fairly effortlessly (fortunately) peeled off the substrate. The back of the paint film was entirely covered with loosely bound skim-plaster, and the moisture level was measured between 19 and 22%. The moral of the story is to apply all masonry primers only to dry substrates, but water-based primers especially only to dry and non-friable substrates.