Grout can be an extremely porous material that can harbor dirt, mold, mildew, and bacteria, which is why many artists opt to seal their work to prevent stains, etc. Over time grout may fade in color from direct sunlight, and many sealants also provide UV protection.
Sealing is recommended for functional mosaics, such as shower inserts or tabletops, but for artworks and nonfunctional mosaics whether to seal or not is a personal choice.
If your project is going outside, will be exposed to spills, moisture, and the elements it is advised that you seal the grout, unless you heeded the earlier advice to purchase grout that already contains sealants. Check the manufacture’s instructions for recommended sealers. We recommend using a penetrating sealer. This will soak into the grout instead of lying on the surface and will provide a longer-lasting result. However, once the sealer has had time to penetrate the grout it should be thoroughly wiped off the surface of your tesserae. Sealant can create a cloudy haze on your tiles especially glass, glazed tiles, etc. Anything that is not porous.
Sealing should be done after the grout has been allowed to properly cure, most manufacturers recommend at least 72 hours after grouting. Many sealers have a lifetime 10 or 20-year warranty, even if this is the case we recommend periodic resealing. Once every 1 or 2 years to keep it protected.
Often we are asked whether you should pre-seal vulnerable materials before grouting such as unglazed porcelains, etc. The answer is almost always no! That being said it depends on the material. Most commercially manufactured unglazed porcelain such as Cinca are very dense and high fired. This means that they don't absorb water, therefore they will not be susceptible to staining, and they also will not absorb the sealer. If there is colorant left after grouting it is typically on the surface and will clean up with vinegar. If the piece that you are grouting contains pieces that absorb water they will need to be sealed. This includes low-fire unglazed ceramics, marble, and other forms of natural stone. To test if the material absorbs water, drop a spot of water on your piece and watch if it soaks in. If it does seal it or protect it in some way such as covering with painter's tape.
