Sealing natural stone means applying an impregnating sealer, a liquid that soaks into the pores and repels water and oil. A spill then stays on the surface long enough to be blotted. The Natural Stone Institute puts it plainly in its care and cleaning guide: sealing makes stone more stain resistant, not stain proof.
Whether you need it depends on the stone and on the individual slab. Marble usually benefits. Quartzite ranges from dense varieties that barely absorb to porous ones that should be sealed. Most granites benefit, and a few do not. A simple water test, described below, tells you which group your counter belongs to.
One note before the details: Stonhaus Design fabricates and installs countertops, and we do not sell sealing as a service. This guide is here so you can do the job yourself or brief whoever you hire, and so you can weigh upkeep while you are still choosing a stone.
Which stones absorb, and which barely do
| Stone | What it is | Does it need sealing? | What sealing will not fix |
|---|---|---|---|
| Marble | Mostly calcium carbonate, softer and acid-sensitive | Usually yes; follow the product label and the water test | Etching from lemon, wine, tomato and vinegar |
| Quartzite | Sandstone fused under heat and pressure, almost all quartz | Depends on the slab: dense varieties may need little or none, porous ones benefit | A mislabeled slab: if it etches, it is not quartzite |
| Granite | Interlocking minerals formed from magma | Most granites benefit; some dense ones need no sealer or a single coat | Hard-water buildup and dullness around the sink |
If low upkeep is a priority, ask for the water test on the actual slab before you buy, whether you are looking at marble or at quartzite slabs in Charleston. Two stones with the same name can behave differently.
The water test that decides whether to seal
- Pick the spots that work hardest: beside the sink, near the range and along the front edge. The institute’s quartzite care article notes that stone darkening around the sink is the best sign a sealer is needed or wearing out.
- Pour a small puddle of hot tap water on a clean, dry spot and leave it for 10 to 20 minutes.
- Wipe it off, dry the area, and let it air-dry for about 10 minutes.
- Look at the spot. If it is darker than the stone around it, the stone is absorbing and is due for sealing. If nothing changed, leave it alone.
The timing follows advice from a stone care specialist in the institute’s granite care article and from geologist Karin Kirk’s porosity test. The same check works on marble, quartzite and granite, and it is worth repeating every few months at the sink.
Marble: seal for stains, manage etching separately
Marble benefits from a current sealer in kitchens, vanities and showers, where oil, makeup and soap would otherwise soak in.
How long a coat lasts depends on the product: the restorers interviewed for the institute’s marble care article mention sealers that last more than ten years and others that last six months, so read the label rather than a rule of thumb.
No sealer stops etching. Acids react with the calcium carbonate in marble whether the stone is sealed or not.
You manage etching with habits and with the finish: a honed surface hides it better, as our comparison of honed and polished marble explains, and the daily routine that keeps marble clean is covered in our guide to keeping marble countertops free of stains.
Does quartzite need to be sealed?
Sometimes. Quartzite starts as sandstone, and heat and pressure fuse the grains into a dense rock. How tightly they fused sets the porosity. In its guide to quartzite, the institute names Taj Mahal and Sea Pearl as highly metamorphosed stones with tightly bonded minerals, and White Macaubas and Calacatta Macaubas as more porous varieties that benefit from sealing.
Both of the tightly bonded examples are on our materials pages, Taj Mahal and Sea Pearl, and each lists periodic sealing as part of its care. The water test on your own slab is still the final word.
Quartzite also has a labeling problem. The same guide explains that marble and dolomitic marble are sometimes sold as quartzite, and that a stone which etches from lemon juice or vinegar is not real quartzite.
The same guide warns that some stones sold as quartzite turn out to be marble or dolomite. If your “quartzite” etches, care for it like marble, and use the two simple tests for a slab sold as quartzite before you buy.
Do granite countertops need to be sealed?
Most do, though not all. Experts interviewed for the institute’s granite care article recommend sealing most granites. They also note how wide the range is: a dense stone like Absolute Black may need no sealer or a single application, while a porous one like Juparana Columbo may need four to six applications to be sealed properly.
Is granite waterproof, then? Not in the sense of absorbing nothing. Some slabs take in water and oil, which is exactly what the water test shows.
The good news is that granite is not sensitive to most household acids, so sealing it is about stains only. If you are still choosing a slab, the water test is worth asking for on granite countertops in Charleston as much as on marble.
One thing sealer will not prevent on granite is hard-water buildup. A stone care specialist quoted in the same article warns that water left around the faucet starts to calcify and turn white, and recommends a neutral cleaner once a week to keep it away.
How to apply an impregnating sealer
- Choose an impregnating (penetrating) sealer labeled for your stone. For kitchen counters, the institute’s care guide says to make sure it is non-toxic and safe for food preparation areas.
- Clean the counter with a neutral stone cleaner, rinse, and let the stone dry completely. Sealer cannot soak into pores that are full of water.
- Apply it the way the label says. The granite care article describes the usual routine as spraying the product on and then wiping off the excess.
- Keep to the label’s dwell and wipe-off times, so no excess dries on the surface and leaves a haze.
- Give it the full cure time on the label before the counter goes back into use.
- Repeat the water test on the same spots to confirm the coat took.
Some impregnating sealers need two applications at installation and then last for years, a designer told the institute in an article on natural stone in busy kitchens. He adds a warning worth keeping: do not leave a spill overnight and expect the sealer to work.
How often to reseal
There is no universal calendar. The institute’s quartzite care article says many modern sealers protect stone for one to ten years.
Its granite article says a good sealer can last up to ten years when the stone is cleaned with the right products. The same source names improper cleaners, along with sunlight and air, as what breaks a sealer down.
So the practical schedule is the water test, not the date. Test beside the sink every few months; reseal when the spot darkens. A stone that passes can be left alone, and adding coats to a stone that does not absorb them only leaves product sitting on the surface.
Before you reseal: a final checklist
- Did the water test actually darken the stone? If not, skip sealing for now.
- Does the stone match its label? If it etches, treat it as marble.
- Clean surface, completely dry?
- Product: an impregnator labeled for this stone and, in the kitchen, for food areas.
- Has daily cleaning switched to a neutral stone cleaner, so the new coat is not stripped?
- Is the problem a stain at all? Etch marks, hard-water rings and scratches need a different fix.
If you are still choosing stone and want one that asks little of you, start with the dense quartzites and test the slab you like. You can see the quartzite countertops we fabricate in Charleston and ask for that test on the slab itself.
