Why Does Water Migrate through an Acrylic Sealer?

Question:

We applied a dry-shake color hardener to a concrete floor slab and then sealed with a solvent-based acrylic sealer. Why does water when left on the surface of the floor seep through the sealer?

Answer:

This is actually a good thing as long as the wet spot disappears completely. It indicates that the sealer was applied properly. Most thin-build acrylic sealers (under 2 mils thick) are chemically designed to be breathable, which means they have permeability. This allows gas and water to move both ways through the sealer. As can be seen in this photo of the floor, the sealer initially causes the water to puddle, and then over time the water migrates through the sealer. This is the process of osmosis – the movement of material from an area of high concentration to an area of low concentration through a permeable membrane. Once equilibrium is reached, the process stops. Evaporation then begins and the water, in gas form, eventually evaporates back out of the concrete through the sealer.

Acrylic sealers, when applied properly, provide a good barrier of protection and allow time for spills to be cleaned up. If left on the surface, the spilled material will migrate through the sealer. This is not an issue with water, but oil and organic material can permanently stain the underlying substrate, which can be a bad thing. Acrylics provide a great cost-to-performance ratio, which is why we see them used so often. Do not make the common mistake of applying more sealer than needed. This is counterproductive when it comes to acrylics and diminishes performance.

If you are looking for better resistance to water migration, consider waxing the surface to help reduce the permeability or apply a high-performance coating (polyurethane, polyaspartic, epoxy or polyurea) that is 100%-impermeable to start with.

Migration of water through a permeable acrylic sealer leaves a temporary wet spot.