Scratch coat mix ratio I swore by for a decade, now I flip flopped

I always mixed my scratch coat at 3 parts sand to 1 part Portland, straight from my old foreman. Then a job outside Cleveland last fall kept cracking no matter what I did. A rep from the supply house asked to see my mix and said that ratio was too lean for their local sand. He talked me into 2.5 to 1 with a handful of lime tossed in, and I thought he was full of it. First wall I did with that held through the winter freeze and didn't show one hairline crack. I'm still not total sold, but I've used it on two more jobs with the same result. Anyone else had to adjust their scratch coat for regional sand?

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1 Comment

murphy.linda7h ago

The "too lean for their local sand" bit is what got me. That sand is probably finer or more rounded than what you're used to, so it needs more paste to actually fill the voids and grab hold. Portland isn't a one-size-fits-all binder, and lime adds that little bit of workability and self-healing that straight cement just doesn't have in colder climates. I've seen guys fight the same wall for weeks before they finally gave in and adjusted, so I'd say your supply house rep actually saved you a headache.