We have a semi-exterior wall with some spots where the paint is peeling and what appears to be dry, powdery plaster is falling out. The wall is under a roof so it's not rained on directly, but otherwise it is exposed to humidity and hot to freezing outside temperatures here in Virginia. It's a brick wall and I've seen no evidence of lathe. The other side of the wall is completely exterior and painted.

What's the best way to stop this and repair it?


1 Answer 1


The blisters in the photo are due to moisture (water vapor) being forced out of the from the opposite side or from the ground. Moisture permeates through the plaster in the path of least resistance, such as a fissure or hairline cracks. If the surface has been painted the water will actually blister the paint as it continues through the wall. The powdery residue you notice is a combination of minerals left behind when the moisture evaporated and whatever effluence it picked up traveling through the plaster. To solve the problem first eliminate the cause of the moisture. This could be any thing from a wayward sprinkler that splashes water against the walls surface to gutters saturating the ground that is against the wall. If the wall is in contact with the soil, excavate to the footing clean the surface and when dry apply a concrete sealer like Dryloc. To repair the blisters wire brush the loose plaster scrape any flaking paint, apply a concrete adhesive to the surface (not critical to the repair, but it helps), and apply a plaster patch as needed. It will blister in another location if the water source isn't corrected.

  • Thank you for your help. We've done a lot of leak repairs and hopefully have gotten them all taken care of. Can you point me to a plaster patch how to that is appropriate for exterior-isn walls like this? Sep 28, 2015 at 15:02
  • The local big box stores will have a large selection for plaster patch. Look for a product that is labeled for exterior use. You can choose from a powder patch that will need water mixed into it or a pre-mixed plaster that is ready for use. Looking at your photo you'll need about 7 lbs. of powder or 2 qts. of ready mixed patch.
    – ojait
    Sep 28, 2015 at 15:08

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