Exterior Plaster Water Damage
Whenever you see bubbles or blisters forming on exterior stucco walls,
there's a good chance and I mean a really good chance that moisture will
have accumulated in the stucco and remained there for a long period of time,
eventually creating bubbles or wall blisters like these.
Moisture can come from the ground, air or even from parts above the damaged area.
The best way to fix this type of damage will be to scrape off the
powdery substance that was the exterior coating. Then it wouldn't be a bad
idea to grab a wire brush and scrape as much of the loose mortar or plaster
off as you possibly can.
Now for the bad news, if you start to see stucco lathing wire or building paper, then the damage might be a little more severe than you anticipated. If this is the case, the paper or wire might be damaged also and will need to be removed and replaced.
After this is done, you can re-stucco the damaged area and fill all of the low spots in with stucco mortar. After you've let that dry for a few days, you should be able to apply the color coat stucco finish.
If you're going to paint this area, let it dry for at least seven days, unless paint or building product manufacturer suggests otherwise. Always follow the manufacturer's instructions.