Mosaic grout

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The purpose of sealing grout is to make it water and mildew resistant, and to help keep out dirt. Many mosaic articles and books say you must always seal grout. I don't always agree. In my opinion, indoor wall-hanging mosaics displayed in dry environments don't need sealing. I simply don't bother because my indoor wall hangings don't get wet or dirty. However, indoor mosaics exposed to dirt and moisture (e.g., tabletops and backsplashes) should, indeed, be sealed.

Grout on all outdoor mosaics should always be sealed for maximum protection from dirt and weather.

There are two basic types of grout sealers: 1) Penetrating, and 2) Membrane forming.

Penetrating Sealers: These sealers soak into the grout and leave deposits when the base liquid evaporates. The deposits, typically latex or silicone, fill the voids in the grout, which then helps the grout resist dirt, grease, and liquid. Penetrating sealers usually don't change the grout color, except to slightly darken it. Higher quality sealers typically result in less grout discoloration.

Membrane-forming Sealers: These sealers don't soak into the grout; instead, they stay on the grout surface and harden, usually to a glossy finish (i.e., they form a thin membrane on the grout surface). Membrane-forming sealers are typically used in grout dyes to purposely change the grout color.

Note that sealers don't provide 100% protection in all conditions. Read product labels carefully under the heading of "Limitations" and you'll notice that they typically say "repel" or "resist," which aren't the same as "provides complete protection." Don't expect sealers to prevent staining in all circumstances.

I prefer a high quality penetrating grout sealer on my glass mosaics. My favorite is TileLab SurfaceGard Penetrating Sealer made by Custom Building Products. It's water based, easy to apply, cleans easily off glass tesserae, and repels dirt, water, oil, and stains. I apply it to my mosaic tabletops and trivets using a soft paintbrush to spread it over the entire surface. Two minutes later after the sealer soaks into the grout, I use paper towels to wipe excess sealer off the glass tesserae. Then, after another two minutes, I use a clean towel to buff off any remaining residue. If you wipe the sealer off the tesserae within a few minutes after applying it, there's usually no problem with it dulling or damaging the glass.

Although the product label recommends two coats, I tested it and found that one coat is usually enough for my indoor mosaic tabletops and trivets. When first trying the product, I let the sealer dry for two hours and then applied a drop of water to a grout line. The water beaded instead of soaking in, indicating the grout was adequately sealed. So, I use only one coat for my indoor mosaics that need stain and water resistance. However, for all outdoor mosaics, I always apply two coats.