Originally Posted by mmdevinc
I wipe the stuff off, and 24 hrs later it's back. No one seems to know what this stuff is, or why it happens. One thing is for sure, it's not mold. This is happening in all 4 bedrooms upstairs, some to a higher degree than others. We used premium paint when we painted. We used the same type of paints on the main floor with no problems. Any suggestions would be greatly appreciated.
OK...process of elimination may be the best thing to determine what's going on here. To begin, how do you know it's not mold? Have you tried to wash it down with a mild bleach or peroxide solution? If so, did the white powder return - if that's the case, you're right, it's probably not mold.
As long as we're experimenting, try washing with a 50/50 vinegar wash on an area and see what happens. If that takes care of the problem, and the chalky appearance doesn't return, you may have an efflorescence problem (especially if these are plaster walls) and a thorough acid wash (mild acid) should take care of it...A vinegar wash should
also take care of laitance and saponification. If any of these are the case, a detergent wash may actually aggravate the problem more. But if it were efflorescence, laitance or saponification, I think there'd be an easily identified caustic "burn" to the paint in the form of blisters, loss of adhesion or discoloration beyond the white haze.
So, if bleach, peroxide or vinegar doesn't do the trick, you've probably got a condition called "hazing" or "frosting" - which is actually caused by one of the components of the paint itself. Frosting is a condition where un-bound, or loosely bound, calcium carbonate "floats" to the surface of a paint film caused by a number of possible factors. It mostly appears on medium to dark tone paints (it happens on lighter color paints also, it's just not very noticeable) - and can be pretty stubborn to remove.
Untreated, frosting can also "bleed" through subsequent coats of paint or primer and just keep re-depositing itself on the surface. Severe cases can actually affect adhesion of the paint as well. While this is not a definitive test, a common or tell-tale sign is if it kind of disappears when washed with just clean, plain water - only to reappear in a few minutes or hours.
The reason for this is kinda simple. Calcium carbonate, a commonly used pigment in paint, is actually baking soda. If you mix a spoonful of baking soda into a glass of water, you'll notice the baking soda turns clear upon stirring. If the water is allowed to evaporate, there remains a white, opaque crust in the bottom of the glass which, of course, is the baking soda. Kinda like a wet t-shirt - transparent when wet, opaque when dry...
IF this is the case, the treatment is not difficult but it's going to require priming and repainting...but not priming with a typical, white primer. You're going to want to use a clear, resinous product such as Zinsser Gardz or an acrylic masonry sealer such as Seal Krete (both are found at most paint stores - not necessarily the big boxes). Usually once primed with either of these two products, you're safe to go with your choice of finish paints.
Finally (and, again, IF this is the case), don't be real quick to blame the last coat of paint applied. This very possibly was caused by any of the previous applications of paint or primer over the years.
I hope this info is helpful - good luck.