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Community and Q&A

Rust Stains on Concrete Stucco

Bruce_Davis | Posted in General Questions on

Three-year old house. During framing, I noted to builder a few of the nails holding up Zip OSB were rusting. He had no explanation or concern.

Fast forward a couple of years and I have what appears to be rust stains on exterior of concrete stucco. See picture. Maybe a dozen different spots. Stucco contractor came out recently. Said it was caused by dirt flying in buckets during mixing. 

They chiseled out the offending spots and patched.  Did not appear to be any nails under the originating spots. Nothing to be concerned about.

Is there any agreement/disagreement on the assessment? I tried cleaning spots with household cleaners before painting. Seem to be stubborn stains on the Sherwin-Williams Duration . Is it OK to paint over stains?

Related question. I see some rare rust stains on concrete driveway & patio. Same thing as stucco issue? Thanks.

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  1. Expert Member

    It doesn't take much iron at all to create a disproportionately sized stain. It can be something as small as a flake of metal that comes off a mixing paddle, or a trowel during the installation. The same thing can happen on concrete with some types of fertilizer if a pellet is allowed to set in one place too long, and gets wet from sprinklers or dew.

    What's interesting is that it appears to be originating from a small void in the stucco. Were the others similar?

    1. Bruce_Davis | | #2

      I didn't notice any such void on the others. Thanks.

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