Grout Release is used to prevent staining or irreversible damage to certain types of stone. Any type of stone that absorbs moisture at a fast rate should be protected before grouting. The problem lies in the grout that gets absorbed into the stone and then dries before it has been washed with clean water. The "burning" reaction goes below the surface leaving a discolored tile. Slate and unsealed saltillo pavers are a must when it comes to needing a grout release applied before grouting. Some high quality sealers can be used as a "grout release" but can be a costly alternative to the less expensive grout release products.
Application can be by sponge,sprayer or roller and care should be used to keep the joints free of grout release. Generally a 30 minute waiting period before applying grout allow the release to form a film that will keep the grout from sticking to the surface. Some types of travertine with a "chiseled edge" may need a grout release applied to the edge only allowing any "unfilled" holes in the field area to be filled with grout and not be washed out due to excessive grout release filling those areas.
Try a small area to get a feel for how the grout is washed off the surface. I have found that slate and saltillo pavers are a very time consuming material to grout even with grout release. You can also try using hot water to wash the grout which helps. Keep your grout water changed frequently. I like to use a clean 5 gallon bucket of water every 20 square feet for floors. The idea is to wash the area leaving very little residue to dry on the surface. Only a very light film of grout "haze" should be noticeable which should clean up easily with a final wash using clean water. Taking the precautionary steps "before" will provide excellent results that will leave your tile project looking clean and professional.
