This Summer, only part way done, I’ve encountered three homes where someone had parged over paint.
If you were a mason in masonry school, I’d hope there’s an exam question that asks about proper preparation for parging. One that sort of mirrors the instructions on a bag of parging cement – clean the surface of all foreign matter!
Cement doesn’t stick to paint.
Yes, you can spread fresh new parging cement against a painted surface, and if no one touches it, it will stand as a thin sheet of cement. Until something happens, like it gets hit, or the ground below it heaves, whatever. Then it cracks, then you get that hollow delaminated sound, telling you the outer layer of parging cement isn’t connected to the foundation.
That’s why I usually pressure wash foundation walls before I apply parging cement. Sometimes you can see they’re clean. But often there is a layer of grime, pollution, and organic matter that has accumulated over the years, and yes, sometimes there is paint! Even water-proofing. You want to get all of that off.
Get it all off, that’s the first step, make it a clean surface.
Then you want to hydrate it, so that it won’t absorb any more moisture.
When I show to parge a wall I generally spend the first half hour or more just watering walls, and then as I am parging I am watering the wall just ahead of me. Why? Because I don’t want the foundation to suck out the moisture from my thin layer of parging. Why? I want it to form properly, to last.
And if you check the better practices, if you talk to the companies that make parging cement, they’ll tell you not to parge on days where the temperature exceeds 25 degrees celsius. That means, if the only thing you advertise that your company does is parging, you should be taking days off.
Parging is a sensitive cement, and it’s a joy to work with, to apply every so softly, to carefully work with as it cures. While it doesn’t involve the artistry of a custom fireplace, it does have a certain zen aspect to it, as it requires patience, a practiced touch, and timing. Masonry is a muscle-memory trade, perhaps no where as much as in parging.
But don’t do it over paint!