Waterproof and Moisture-Resistant Coatings

What a Moisture-Resistant Coating Can and Cannot Do

Start with the honest limit, because the marketing on these products oversells them badly. A moisture-resistant coating is a paint film with a tighter, harder, less porous surface than standard wall paint, usually with a mildewcide built into the film so growth is slower to take hold on it. That is genuinely useful. It is not waterproofing in the sense a tile setter or a plumber uses the word, and it will not hold back water arriving from the wrong side of the wall.

Paint fails in a bathroom in one of two directions. Water lands on the surface, sits, and slowly works into a porous or dirty film. Or water arrives behind the film as vapor, condenses inside the wall or on the cold back face of the ceiling board, and pushes the whole coat off in sheets. A good coating solves the first problem well. It does nothing at all for the second, which is a ventilation and sometimes a construction problem, and any painter who tells you a special product fixes it is guessing or selling.

So the sequence we use is always the same: find out which failure you have, fix the cause if it is fixable, prepare the surface honestly, then choose the coating.

The Systems We Actually Use

Bathroom and kitchen wall systems. A washable acrylic in satin or semi-gloss with a mildewcide in the film, over a primer chosen for whatever is underneath. Satin where the wall is uneven and shine would advertise every ripple, semi-gloss where the surface is flat and durability matters more than forgiveness.

Ceiling systems for shower rooms. The shower ceiling is the hardest surface in the house. Warm vapor rises to it and the back of the board is often the coldest thing in the room. Flat ceiling paint is exactly wrong there. We put a tighter film on it and we accept that it will show a little more of the surface, because the alternative is a peeling ceiling.

Masonry and stucco primers on the exterior side of a wet wall. Where a bathroom backs onto a stucco elevation that gets driving fog and salt, the outside face matters. Masonry primer bonds to alkaline surfaces that standard primer cannot handle, and elastomeric coatings will bridge hairline stucco cracks. Elastomeric comes with a real caution: it is a thick, flexible, relatively vapor-closed film, and putting it on a wall that needs to dry outward traps moisture and blisters. We use it where it earns its place and we say no where it does not.

Stain blockers and bonding primers. Rust weeping from a fixing, tannin bleeding out of old redwood trim, a water ring from an upstairs leak, or a glossy old enamel that nothing wants to stick to. These want shellac, oil-based blocker or a dedicated bonding primer, and they are not interchangeable with each other.

The Prep That Makes It Hold

Cleaning is most of it. Bathroom surfaces carry soap film and body oils, kitchen surfaces carry cooking grease, and both are invisible and both defeat adhesion. We degrease, rinse, and let it dry.

Live mildew is washed out with a biocidal cleaner and rinsed, never coated over. It grows through paint, so covering it buys you a season at most.

Soft or spongy board near a tub edge is cut out and replaced with a moisture-resistant panel. Skimming over it is a repair that fails on the same schedule as the last one.

Joints that move get flexible caulk, tooled and given time to skin before anything goes over it. Joints that do not move get filler.

Then the coats go on with real dry time between them, planned around the dew point rather than around our schedule. On a foggy Santa Monica morning an unheated north-facing bathroom can sit close enough to the dew point that a film will not form properly, and a coat forced on in those conditions stays soft and blotchy. We would rather start after lunch.

Call (310) 668-0370 and describe the failure. Peeling in sheets and speckled grey growth are two different problems with two different bills.