White powder on a stone wall, patio or veneer is almost always efflorescence: salts carried to the surface by water, left behind when the water evaporates. The powder itself is rarely the problem. The mistake that ruins stone is reaching for a strong acid because a forum post said so, on a stone that acid dissolves.
So the useful question is not which cleaner is strongest. It is what kind of stone you have, how much water is still moving through it, and how gently you can get the salt off. This guide sorts stone into acid-tolerant and acid-sensitive, lays out the order to try things in, and covers the one cleaner I would link, with its limits. If the white deposit is on poured concrete or a basement wall, the same salt logic applies, and the foundation waterproofing sealant guide covers what to do about the water.
What efflorescence is, and when it matters
The Brick Industry Association’s Technical Note 23A says three things have to be true at once: soluble salts in or touching the masonry, enough water to dissolve them, and pores that let the salty water travel to a surface where it evaporates. Take away any one and the powder stops. The salts usually come from the mortar, grout, concrete, soil or backing, not from the stone itself, and the white shows up most on dark surfaces.
The same note separates two situations. Efflorescence on work less than a year old is “new building bloom,” and it usually fades as the wall dries and weather washes it. Efflorescence that appears or keeps returning after the first year points to water getting in or poor drainage, and it tends to be worst in winter. A National Institutes of Health Division of Technical Resources bulletin on masonry walls says the same: some efflorescence on all new masonry is normal and harmless, while persistent or heavy deposits can mean excessive internal moisture or rising damp that should be investigated. Both sources warn that crystals forming inside the stone, instead of on its surface, can crack it.
That gives you a simple rule. Light powder on a new wall or patio: wait, brush, and let rain help. Powder that keeps coming back: cleaning is only housekeeping, and the water source is the real job.
Which stone do you have
The National Park Service’s Preservation Brief 1 splits masonry by how it reacts to acid. Its list of masonry that acid-based cleaners may be used on includes granite, most sandstones, slate, unglazed brick, unglazed terra cotta, cast stone and concrete. Its list of acid-sensitive masonry, which should get alkaline or neutral treatment instead, includes limestone, polished and unpolished marble, calcareous sandstone, glazed brick and polished granite.
| Surface | Acid cleaner? | Start with |
|---|---|---|
| Concrete pavers, cast stone, concrete block | Only after the gentler steps fail, and only as the label directs | Water, stiff non-metal brush, a paver-labelled cleaner |
| Unpolished granite, slate, most sandstone | Possible after a test patch | Water and brush, then a mild cleaner |
| Limestone, marble, travertine, calcareous sandstone | No | Dry brushing, then water, then a poultice. Ask a stone restorer before any chemical |
| Unknown or mixed stone | Treat as acid-sensitive | Water only, plus a hidden test patch |
If you cannot tell which row you are in, assume the more fragile one. Park Service guidance notes that it is not always clear exactly what a stone is made of, which is why testing on a hidden spot is always necessary.
The order to try things in
- Wait, or dry brush. A stiff non-metal brush on dry stone removes loose salt. Wire brushes and sandblasting are not recommended, especially on brick and soft stone.
- Water and a stiff non-metal brush. Use low pressure. The NIH bulletin warns that washing can draw more salts out of the wall, so the first washing may bring up a fresh bloom and you may need to repeat it.
- A poultice for salt that is deep in the stone. A poultice is a paste of absorbent clay powder (kaolin or fuller’s earth, or even shredded paper) mixed with a liquid. Spread on the stone and allowed to dry, it pulls the salt into itself so it is not redeposited. The Park Service shows one being used to draw salts out of brownstone statuary. This is the safest chemical-free route for acid-sensitive stone, and it is slow.
- A cleaner, tested first. Only now, and chosen for your stone type from the table.
Two timing rules apply to every cleaner. Park Service guidance says chemical cleaning should never be done below 40 degrees Fahrenheit and generally not below 50, because water and cleaners can freeze, and many cleaners simply do not work in the cold. And judge a test patch only after the stone has dried completely. The brief suggests starting a test area as small as six square inches, and ideally letting a successful test patch weather for a month or two before committing to the whole surface, since cleaning can bring up more salts that show later as a second bloom.
A real example: how much cleaner for a 12 by 15 foot patio
Take a 180 square foot paver patio. Alliance Designer Products’ data sheet (revised May 2025) for its Gator efflorescence cleaner, which is labelled for concrete pavers and natural stone and contains no hydrochloric acid, gives these numbers: mix 1 part concentrate with 4 parts water, use 1 gallon of mixed solution per 40 square feet, and test a hidden area of about 4 square feet first. Plan on 180 divided by 40, or 4.5 gallons of mixed solution, which is 0.9 gallon of concentrate. Rinse until the foaming stops, and do not let it dry on the surface. The sheet also says to wait at least 60 days after new pavers are installed, and 48 hours after fresh polymeric sand (our guide to choosing paver joint sand), and that stronger mixes than 1 to 4 can discolor the pavers. I could not link that product: the only Amazon listing I found for it was sold by a third-party seller with no customer reviews.
The test patch is 4 square feet out of 180, a little over 2 percent of the job, and it is the cheapest insurance on the list.
A paver manufacturer’s guide I read (Western Interlock) uses muriatic acid instead, and it is a good example of why to follow the product label and not a blog. It says to mix “approximately an 11:1 ratio of water to acid, or 20 ounces of acid for every two gallons of water.” Two gallons is 256 ounces, so 20 ounces of acid is about 13 to 1, not 11 to 1. The two numbers in the same sentence disagree. The Park Service, for its part, says muriatic acid is very powerful, can dissolve lime-based mortar, damage brick and some stones, and leave chloride deposits, and should not be used on historic masonry. On a patio of modern concrete pavers it is a judgment call. On old stone, mortar or limestone, it is not.
The one cleaner I would link, and what it is not for
Miracle Sealants Heavy-Duty Acidic Cleaner and Rust Stain Remover, 32 ounce. It is labelled for removing grout haze, hard water deposits, rust stains and efflorescence from tile and non-polished natural stone. Amazon rated it 4.1 stars from 862 ratings with an Amazon’s Choice badge and a 400+ bought in the past month note, sold and shipped by Amazon and in stock when I read it. The listing says it is non-returnable because of hazardous materials shipping, and it describes the product as a strong acid for textured, non-polished surfaces. Reviews are mixed: 62 percent are five stars and 10 percent are one star, and one October 2026 reviewer reported outdoor flooring damaged, with a white powdery residue, two days after use. That is the failure the test patch and a thorough rinse are meant to catch. Because it is an acid, it belongs in the acid-tolerant rows of the table only. Do not use it on limestone, marble, travertine or any polished stone. The two dilution guides I found for it disagree (one retailer starts near 1 part cleaner to 20 parts water, the manufacturer’s page says up to 10 to 1), so use the ratio on your bottle, wet the stone first, work in small sections, and rinse thoroughly.
Check price on Amazon: Miracle Sealants Heavy-Duty Acidic Cleaner and Rust Stain Remover. This is an affiliate link (see our disclosure for details).
For acid-sensitive stone I am not linking anything. I did not find a consumer product that is both labelled for efflorescence and clearly safe on limestone or marble, and the same maker’s non-acidic stone cleaner points efflorescence jobs back to its acidic one in the sheets I found. Water, a brush and a poultice are the honest answer there, and valuable or historic stone is worth a call to a stone restoration specialist before you try anything else.
Stop it coming back
Cleaning without fixing the water is a subscription. Look for the entry points: failed mortar joints, missing flashing, a patio that slopes toward the wall, gutters that dump at the foundation, or soil piled against stone. The NIH bulletin recommends granular, well-drained backfill behind retaining walls and ground that slopes away from walls, and waterproofing masonry that touches the earth to control rising damp. Cracks that let rain in can be sealed, as covered in our concrete crack filler guide.
Do not seal over efflorescence, and be careful with water-repellent coatings in general. The Park Service explains that a coating can hold moisture inside the wall, and in the case of rising damp it may push moisture higher by slowing evaporation. The Brick Industry Association likewise advises against coatings as the only treatment. If you plan to paint masonry afterward, see our masonry primer guide, which covers alkali-resistant primers and cure times.
Etienne Basson maintains and repairs his own home, and writes practical guides on choosing the right materials and products for repair and finishing jobs: fillers, sealants, adhesives, and similar. He focuses on what actually works for a specific job, not just a spec sheet.