Sealing, staining and etching of stone

At a glance

All from the Natural Stone Institute (NSI) consumer pages, undated, read 6 Nov 2026. General statements, not data for a slab: ask the partner for the absorption result (BS EN 13755) for yours.

Two problems, not one

People say ‘the marble is stained’ for two different faults. A stain is a liquid carried into the pores of the stone, where it leaves colour. An etch is a chemical change at the surface. The Natural Stone Institute (NSI) says etch marks are caused by acids left on the stone, that some materials etch the finish without leaving a stain, and that others both etch and stain. On polished calcite stone an etch reads as a dull patch that catches the light differently from the polish around it.

The distinction matters because a sealer helps with only one of them. NSI says the products sold as sealers in the stone industry are impregnators, which do not seal the stone but act as a repellent, and that sealing makes stone more stain resistant, not stain proof. The two sealer makers’ data sheets we read are plainer still: one says the treatment does not alter a stone’s sensitivity to acids and that impregnators do not protect the surface against acid; the other says its product will not stop acid etching. A stone that etches will still etch when it has been sealed.

Which stones etch, and what does it

NSI sorts stone into calcareous stone, composed mainly of calcium carbonate, and siliceous stone, composed mainly of silicates such as quartz, feldspar and mica. Calcium carbonate is sensitive to acidic solutions, so NSI recommends mild, non-acidic cleaners for it. Siliceous stone is generally resistant to most acids found in kitchens, though NSI still advises against acidic cleaners because such stones may contain trace minerals that are acid sensitive. Onyx is a cave deposit that NSI calls vulnerable to chemical and abrasive attack.

The drawing sets the typical pH of common spills, from a US Food and Drug Administration table of April 2007, against a neutral cleaner. NSI says only that calcite is vulnerable to ‘mild acids’: none of the sources we read gives a pH at which etching starts, so the drawing shows where common liquids sit and makes no claim about a threshold. NSI adds that products containing lemon, vinegar or other acids may dull or etch calcareous stones, that scouring powders can scratch, and that many rust removers contain hydrofluoric acid, which attacks silicates and so every stone, granite and quartzite included.

What an impregnator does, and what it does not

• What it does. A penetrating sealer, or impregnator, soaks into absorbent stone and lines the pores, so that water-based dirt, oil and grease are held back and stains are slower to form (maker’s data sheet; the Singapore Building and Construction Authority, BCA, says the same). It buys time to blot a spill. It works only where the stone is absorbent, and the BCA says a compatibility test should be made, since compatibility varies from stone to stone.
• Types. The BCA says impregnators are solvent-based or water-based: solvent-based penetrate quickly and deeply but give off volatile organic compounds and a strong smell; water-based are more environmentally friendly with little or no smell. One data sheet recommends the solvent-based product for dense limestone, for honed or flamed granite and where the highest water repellency is wanted, such as a shower.
• How it is applied. One data sheet asks for a clean, dry, absorbent surface, a surface temperature between 10 and 25 °C, floor heating switched off, two coats two hours apart, residues towelled off within 15 minutes, and 48 hours before the treatment is fully effective, with no covering before then. The BCA asks for a test of the impregnator on the stone first, and for treated stones to be given time to cure before they are fixed. It also warns that treating the back of the stone may interfere with the adhesive, so that a pull-off test or the adhesive maker’s advice is needed.
• How long it lasts. The published figures do not agree and none is for a hotel floor. One data sheet gives about four years, depending on the stone, use and maintenance regime, and asks for surfaces under increased wear to be touched up at least once a year. NSI’s 2006 position on countertops speaks of a quality sealer with a life of ten to fifteen years. Cleaners can shorten the life: the data sheet says residues of cleaners may affect the treatment, so rinse well.
• Slip. HSE says the slipperiness of flooring can change significantly with the installation process, for example grouting, cleaning, burnishing or polishing, and with inappropriate maintenance and wear, and that the data needed should relate to the floor as finished for its intended use. One sealer data sheet claims treated surfaces are ‘harder and less slippery’, which is a maker’s claim we could not check. So test the treated, finished sample wet, as the floor will be used. See slip resistance for floors.
• Film-forming (topical) coatings. The documents we read cover impregnators only. If a partner proposes a coating that forms a film on the surface, ask for its slip result on a sample, its wear life under traffic, and how it is stripped and renewed.

A cleaning regime to agree

A cleaning regime is part of the specification, because it decides how the stone looks in year three. The tasks below are those the sources agree on. The frequencies are not sourced: the sources say ‘frequently’ or ‘after each use’, and give no schedule for a hotel. The Stone Federation Great Britain code of practice for internal flooring, which has a section on cleaning, maintenance and protection, is shown as currently unavailable and under revision, and we have not read it. Put the intervals in brackets, and agree them with the partner and the operator’s housekeeping or stone care contractor.

• Neutral means pH 7. The BCA asks for clean water and a neutral (pH 7) cleaner, a thorough wipe with water to remove all traces of cleaner, and buffing dry, because water left to dry on its own may leave etch spots. NSI asks for a neutral cleaner, stone soap or a mild dishwashing detergent in warm water, and warns that too much cleaner leaves a film and streaks.
• Blot, do not wipe. NSI and the BCA both say to blot a spill with an absorbent material, since wiping spreads it, then flush with water and rinse. The BCA also advises protectors under furniture, and caution with rubber or jute-backed mats, which can stain the stone themselves.
• Etch marks. NSI says light etch marks on marble can be rubbed out with marble polishing powder on wet stone and a damp cloth or a buffing pad on a low-speed drill, and that a stone restorer should deal with marks that will not come out. Stone Federation GB says dull, etched or marked surfaces can often be professionally refinished.
• Choose the finish and the client’s expectations too. NSI says anyone choosing marble for kitchens and bars should accept the maintenance and the patina. See natural stone finishes for how a honed or textured face hides etching.

Specifying sealing, and protecting the stone until handover

The BCA recommends final polishing and sealing of newly installed or rectified stone before handover, to enhance the shine and provide stain protection, and asks for completed stonework to be protected from traffic, sunlight and rain, with the protective cover removed only just before handover. BS 5385-3:2024, the current code of practice for stone and terrazzo floors, covers design, installation and maintenance; it replaced BS 5385-5:2009, which BSI shows as withdrawn on 30 May 2024. We read BSI’s listings and not the texts. Write these points into the specification and hold the partner to them:

• The stone, finish and edges, approved on a sample cut from the production slab (see stone sample, slab and book-match approval).
• The impregnator, chosen by the partner for that stone and finish, applied after a test patch, and the cure time before traffic.
• Protection until handover, and what the final clean and treatment are.
• The cleaning and maintenance regime, handed over in writing with the operations pack (see handover and O&M pack).

Natural stone [type, finish] to be treated with a [water-based / solvent-based] impregnator [product reference] chosen by the stone partner for this stone and finish, applied to the face and exposed edges [and cut edges of cut-outs] on a clean, dry, absorbent surface within the sealer maker’s temperature range [underfloor heating off during application and cure]. A test patch on a spare piece of the approved slab to be approved before treatment. No sealer on the back of the stone unless the adhesive maker agrees in writing. Treated sample to be tested wet by the pendulum method [BS EN 16165] with the result issued before work starts.

Completed stonework to be protected from traffic, direct sunlight and rain with [breathable / suitable] protective covering, upturned at the edges, kept in place until [ ] days before handover. Final clean and treatment to be carried out by the stone partner or its approved contractor before handover. Damage or etching found at handover to be made good before acceptance.

The partner shall issue a written regime for each space: cleaner type and pH [neutral, pH 7], dilution, mop and machine type, rinse and dry method, spill procedure, and products that must not be used [acidic cleaners, scouring powders, rust removers containing hydrofluoric acid]. Frequencies [ ] agreed with [operator / stone care contractor]; re-treatment every [ ] months or as the sealer maker advises.

Standards and sources

• Natural Stone Institute consumer pages (care, stains, sealing position adopted 8 Nov 2006, marble, limestone, granite); Singapore BCA, Natural Stone Finishes (April 2018); HSE, Assessing the slip resistance of flooring (2012); Stone Federation GB, Natural stone in your home; two sealer makers’ data sheets; US FDA pH table (April 2007): read 6 Nov 2026. Stone Federation GB Code of Practice for internal flooring: ‘currently unavailable, under revision’; not read.
• BS 5385-3:2024 (Current, 31 May 2024; replaced BS 5385-5:2009, withdrawn 30 May 2024); BS EN 13755:2008; BS EN 16165:2021; BS EN 12670:2019: BSI Knowledge listings, 6 Nov 2026. Named, not read.

Questions we are asked

Does sealing stop marble etching?

No. Etching is acid reacting with the calcite at the surface. NSI says impregnators act as a repellent and make stone more stain resistant, not stain proof, and the two sealer data sheets we read say that the treatment does not protect against acid or stop acid etching. A honed or textured finish hides etching better; see our finishes sheet.

Do granite and quartzite need sealing?

NSI’s 2006 position on countertops says most granite countertops do not need sealing, though many benefit from further reduction of moisture migration, and that in many cases it makes sense to seal marble and granite. For a hospitality floor, ask the stone partner for the absorption result and its recommendation for the actual slab, and test a patch.

How long does an impregnator last?

The published figures disagree. One data sheet says about four years, depending on use and maintenance, and touch-up at least once a year on surfaces under increased wear; NSI’s position on countertops speaks of ten to fifteen years. Neither is for a hotel floor. Ask the partner and the sealer maker for a regime for the use.

Does sealing change how slippery a floor is?

It can. HSE says slipperiness can change with installation processes such as cleaning and polishing, with maintenance and with wear, and that test data should relate to the floor as finished for its intended use. Test the treated sample wet, as the floor will be used.

Can an etched marble floor be repaired?

Often. NSI says light etch marks can be polished out by hand with marble polishing powder, and that a stone restorer should deal with marks that will not come out. Stone Federation GB says dull, etched or marked surfaces can often be professionally refinished.

What should a hotel use to clean a stone floor?

A neutral (pH 7) cleaner in clean water, rinsed and buffed dry, with spills blotted at once, mats at the entrance, and no acidic cleaners on calcite stones. The frequency should be agreed with the stone partner and the operator’s housekeeping or stone care contractor.