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How many applications of sealer are needed?
For some stones that are more porous than others, one application of sealer/ impregnator may not be enough. But how will you know? On mercantile granites that need sealing, at least two applications are recommended. Very porous mercantile granites, sandstone, quartzite, etc. may require three or more applications. When sealer can no longer be absorbed by the stone, the stone is adequately sealed.
How long will it last?
There is no absolute rule of thumb when it comes to the durability of any sealer. Generally speaking, most quality impregnating sealers interior will last 2-5 years or more. Environment plays a big role. Stones exposed to intense heat or direct sunlight will probably need to be re-sealed more often.
When is it time to reseal?
To find out if your stone is perfectly sealed, spill some water on it and wait for approximately half an hour, then wipe it dry. If the surface of the stone did not darken it means that the stone is still perfectly sealed. Be sure to test various areas, especially those areas that get more use.
Contrary to what your perception may be when you hear the word sealer, most sealers for stone are below-surface products and will not alter in any way, shape or form the original finish produced by the factory. They will not offer protection to the surface of the stone, either. They will only go inside the stone by being absorbed by it (assuming that the stone is porous enough to allow this to happen) and will clog its pores, thus reducing its natural absorbency rate. This will help prevent possible accidental spills of staining agents from being absorbed by the stone.
There is no blanket rule when it comes to sealing natural stone. Marble (especially all those mercantile marbles that are actually compact limestone) and travertine are NOT very porous. If you don’t believe this, spill a few drops of water, say, on a polished travertine tile, and observe how long it will take to be absorbed (the area under the water would become darker). A very long time, if ever! On the other hand, most granite must be sealed. Granite is indeed more porous than marble and will stain if not protected with a good quality impregnator-type stone sealer. Some “granites” are so porous, that no sealer will do a satisfactory job at sealing them 100% for a long time.
Sealers for stones, which are below surface, penetrating-type sealers (better referred to as impregnators), are designed to do one thing and one thing only: clog the pores of the stone to inhibit staining agents from being absorbed by it.
In fact, in some instances, “weird” problems that may appear to be etching on “granite” countertops turns out to be that the residue of sealer left on the surface of the stone (nothing went inside it) was being etched, certainly not the stone. In these instances, once the sealer is professionally removed, everything is fine.
Note: Sometimes, marks of corrosion (etch marks) that an acidic substance will leave behind when coming in contact with the surface of some stones may look like water stains, or water rings, but they are neither stains, nor were they generated by water. The deriving (surface) damage has no relation whatsoever with the porosity of the stone (which determines its absorbency), but is exclusively related to its chemical makeup. No sealer in the world will do anything to prevent this. See the section on Stain Management for more information.
Color Enhancing Sealing
While impregnating sealers will not alter the appearance of your stone, on tumbled marble, low-honed finished limestone and travertine, honed (black) granite, etc. a color enhancing (impregnating) sealer will protect the stone while bringing out its color, giving it a wet (i.e. darker, not glossy) look. It will at the same time provide good protection from water based staining.
Sealing Grout - Clear or Color Sealing
Grout is porous and will absorb liquids which can potentially stain. Sealing your grout provides a protective barrier that not only protects it from stains, it makes routine cleaning and maintenance easier. Grout can be sealed with a clear sealer or it can be color sealed. Color sealing has the added advantage that it allows you to completely change the color of your grout whether it is just for a new look or to cover up stained and discolored grout.