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Beware of Applying Acrylic Sealer Too Thickly
Here's a photo of a self-leveling overlay that I stained and sealed with a 30%-solids acrylic sealer. The slab is in a screened porch under a deck. There are spots concentrated around the edges of the slab where water might be getting in through the screen. I thought the spots were left from the water, but I tried scrubbing them and they won't come off. They do disappear when they are wet.
Have you ever seen anything like this? Normally when my sealer has moisture issues, it appears cloudy. Some of these spots almost appear as if the sealer has cracks in it. It's weird!
The acrylic sealer is actually fracturing. Typically this occurs when the sealer is applied too heavily, as it appears to be in this case. Acrylic sealers are designed to go down very thin. Even with two coats, the thickness should be only about 1 to 2 mils. To give you some reference, a credit card is about 120 mils thick.
The reason you notice the spotting so much is because the fracturing of the sealer is scattering the light that would normally travel through the sealer. When light can travel through a sealer without interference, you'll see a nice, clean reflection. Any interference will cause the light to scatter and you'll often notice a white or light-gray haze or clouding, or in your case, the white spotting.
I am not a fan of acrylic sealers with a solids content above 24%. At higher solids, you need to be very careful to apply the sealer thinly. Acrylics are tough stuff, but they are prone to cracking once they exceed 1 to 2 mils in thickness. The substrate they are applied to also plays a part. Self-leveling overlays are pretty dense and usually have a polymer-rich paste on the surface. Unless this paste is sanded off prior to staining or sealing, you run the risk of limited sealer penetration and compromised adhesion. Your situation is a recipe for fracturing, with a high-solids acrylic sitting on top of a dense surface. Opening the surface by lightly sanding is usually enough to allow penetration.
If you like the high-gloss look of an acrylic but want to avoid this problem in the future, consider using a two-part polyurethane, which is designed to go down at 3 to 5 mils. Or spread out the acrylic more thinly to achieve a 1- to 2-mil thickness and then apply multiple coats of a wax-modified finish coat or sacrificial topcoat designed for concrete floors.
Author Chris Sullivan, ConcreteNetwork.com technical expert and vice president of sales and marketing for ChemSystems Inc.
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