Read on and I just may be able to help you out with this nuisance.
To find the best solution, we should first examine the common causes.
Low Quality Sealer
Well, you know the old saying that applies to most things and the same goes here. "You get what you pay for." I realize most people don't want to spend alot of time and money to reseal concrete. But when you use a cheap product you're going to get poor results. You can usually get away with it on regular concrete, although it is not giving you the protection you might think. With decorative concrete, it will be very noticeable.
When moisture mixes with sealer before it hardens or cures it turns cloudy white, especially with solvent based sealers. Water based sealers can usually tolerate a little bit of moisture with no problems. Whether the container was contaminated with water, the sealer was applied to a damp surface, or rain got to it, the end result is the same.
Although this is not a problem with the sealer itself, it still requires something being done with the sealer to correct or alleviate the problem. When moisture moves up through the concrete, it dissolves salts along the way. The moisture eventually reaches the surface, evaporates, and leaves salt deposits. The white powder substance, also known as efflorescence, can remain trapped under the sealcoat.
This is the least common cause, nevertheless it still happens and has happened to me before. Certain types of sealers, when applied to thick or puddles get left behind, can have cloudy white spots show up.
Most of the time there is a pretty simple solution, especially with exterior acrylic sealers. Acrylic sealers are soft, breathable coatings that can be "melted" by applying a solvent such as xylene. This is what we call a solvent bath. Just rolling on a thin coat can fix many problems. Eventually the sealer hardens again and the white spots are gone.Test it on a small area first before bathing the whole surface.
Sometimes the only solution is strip the sealer and reapply it. Citrus and soy based strippers work well for most sealers, but it can be time consuming and messy. Because there are so many different types of sealers, my recommedation is to contact the manufacturer of the sealer and ask them what to strip it with.
A few tips to avoid concrete sealer turning white....
Always, always read and follow label instructions carefully.
Make sure the surface is dry before applying concrete sealers.
Do your homework and make sure the type of sealer is appropriate for your surface.
Know the weather forecast 24-48 hours after application. Don't take the chance of rain ruining your project.
Spend the money for a quality sealer. I always recommend contractor supply stores over hardware and big box stores. Not only do they have much better products, but there's usually someone with much more specific knowledge of the products.
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