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Paint Bubbling Over Efflorescence – How To Repair?

6 months ago

About a week ago we noticed our elastomeric paint bubbling as drawn and photographed below. By our best recollection, it was not present the last time we did yard work about a month before. In the interim, we had one day of rain, and washed the deck above this wall, but the deck scuppers are to either side of the bubbling, not above it. It is exposed to a dewy marine coastal environment in Ventura, CA, and this wall is shaded until late afternoon.

As shown, this is an exterior wall, below a vinyl window, and has a sharp upper horizontal edge that appears to be just over the bottom of the plastic stucco weep screed where the underlying block wall is topped by the framing lumber. The stucco extends below the weep screed over the block wall down to the concrete mow strip. There is a few inch gap at the top of the block wall where the waterproofing was not applied during construction just behind the weep screen and bubbling paint.


Only this 3 foot width along the approximately 40 feet of block wall topped with framing is affected. There are three identically constructed windows along this wall, but only the left one is affected. New construction occurred 2004-2005, stucco was first painted 2007 with primer, two coats elastomeric, two acrylic topcoats. Repainted 2019 with a single acrylic topcoat, possibly Dunn-Edwards Evershield.


Inspection of the vinyl window frame after removing the screen and horizontal sliding pane shows the drain holes are open, and the part of the frame bottom vinyl extrusion that is visible has no cracks, acknowledging that I cannot inspect the bottom surface that lies on top of the window framing. There is some mild loss of adhesion of the elastomeric paint to the bottom of the vinyl window frame at the right corner, but this is to the side of the bubbled area. Although I see no obvious separation at the bottom junction of paint to the vinyl window frame and the paint between this and the bubbling below appears well applied to the stucco, it probably wouldn’t hurt to caulk this seam between the paint and vinyl window frame.


This bubbled paint was already cracked in places, in contrast to the surrounding intact paint that does not appear cracked. I poked a hole in the bottom-most area of bubbling and inserted a 2mm hollow straw, but no liquid drained. Then I carefully used a utility knife to cut and removed a 1” wide section of the bubbled paint to unroof and inspect what might be underlying it. Unroofing the bubbled paint reveals a dry mix of powder and small crystals as photographed below. Placing a crystal in a bowl and adding a drop of water causes it to completely dissolve in a few minutes. The powder and crystals easily brushes off to reveal what appears to be intact stucco butting against the underside of the plastic weep screed, which is aligned with the sharp top margin of the bubbled area.


I am suspicious that efflorescence deposits below the paint are causing the bubbling. I doubt the source is the concrete mow strip below traveling up through the stucco, as I would expect to see efflorescence on the mow strip and bubbling along the mow strip unrelated to the weep screed above, both of which are absent. I am wondering whether the efflorescence is coming from the stucco, or transmitted through the stucco from the block wall behind it, as the waterproofing emulsion and membrane used on the block wall terminates just below the bubbling. Given the peculiar alignment of the bubbling with the untreated top few inches of the underlying block wall, I am suspicious that the block wall is the efflorescence source, rather than the stucco, as if it was the stucco, I would expect it to be present at the base of the mow strip as well. The property line site block wall is stuccoed in parts unpainted and other parts only painted with acrylic, not elastomeric, and the only efflorescence I have seen on the site wall is along a narrow several foot width where my neighbor’s soil grade is a few inches above our paved walkway. After a good rain, powdery needle like crystals can extend ¼” on my side here. This further suggests that the efflorescence source is the block wall, as if it was the stucco which is regularly soaked when it rains, I would expect widespread efflorescence all over my block wall, which has not been observed.


If my suspicions are correct, I am uncertain why this is occurring only below this particular window, and where and how moisture is penetrating to the block wall in amounts sufficient for efflorescence to suffuse through the overlying stucco and crystallize thick enough to bubble the overlying elastomeric paint. The interior of this wall is furred out about 2” with framing overwhich it is drywalled and painted, down to the concrete slab, and I have not observed any water damage on the interior wall. I am uncertain if the moisture is wicking up through the stucco from the mow strip below perhaps where caulking to the mow strip is not intact, entering from above between the vinyl window frame and junction with the elastomeric paint, seeping into some undiscovered crack in the bottom of the window frame rather than out the drain holes, or through undiscovered cracks along the lower edge of the weep screed.


I have most of a gallon of 2020 Valspar Exterior Duramax elastomeric color matched to this area leftover that I used to cover a stucco repair elsewhere. I am wondering whether something other than just caulking the junction of the current paint to the vinyl window frame above and concrete mow strip below, and applying two coats of this Valspar elastomeric paint directly over the unroofed and brushed off stucco under the bubbling areas would be recommended to prevent this from recurring.


Thanks in advance for any more expert advice.




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