Three jobs, three bottles
Patio shelves mix ‘clean’, ‘kill’ and ‘protect’ into one promise. The SDS usually names one active. Match it to the job in front of you: biological film, mineral haze, grease, or water getting into already-clean stone.
Biocide (BAC, DDAC, sometimes hypochlorite or percarbonate) is for growth. Acid is for cement and mineral scale on stone that can take it. A water-repellent is the last step, on dry masonry, after the wash has been rinsed away. Oil and traffic grime are a fourth job — alkaline degreaser — and they are not what a sealer or a quat is for.
When the job is algae or black spot
Soft green film on a path that tolerates a quat: benzalkonium chloride (Wet & Forget, Patio Magic, CAS 68424-85-1) or DDAC (Jeyes 4-in-1, CAS 7173-51-5). Spray, leave, let weather do the rinse the label allows. They are not interchangeable 1:1, and neither is a cement remover.
Stubborn black lichen dots often want hypochlorite (Hagesan-style), not another quat. A scrub-and-rinse oxygen powder (sodium percarbonate, Logco Eco) is a third oxidiser — not a leave-on biocide and not bottle bleach. Acetic acid (Algon) browns algae slowly; it is vinegar chemistry, not BAC. Never mix a quat with bleach, and never mix acid with bleach.
When the job is cement haze or mineral scale
Mortar smears and cement bloom on acid-safe brick and concrete are a hydrochloric acid job (Mortar Clen, CAS 7647-01-0). Test a hidden patch. Keep it off limestone, marble, travertine and cement joints you want to keep. Rinse until the water runs neutral.
Acetic acid and phosphoric rust gels are different acids for different stains. Do not reach for brick acid because the patio is green, and do not reach for a biocide because the haze is grey cement.
When the job is keeping water out
Seal only clean, dry, absorbent masonry. Thompson’s Water Seal is n-octyltriethoxysilane (CAS 2943-75-1) in a water-based emulsion — a penetrating silane you dilute from a 50% concentrate about 1+9. It is sold to bead water without a heavy colour change.
Lithofin Stainstop PLUS is a different impregnator: amino-functional dimethyl siloxane (CAS 69430-37-1) at about 15–20% in white spirit, used neat. It also repels oil and it darkens rough and honed stone. The 5 litre tin is the same liquid as the 1 litre tin — a small saving on pack size, not a stronger active. Do not thin it with water, and do not put it on a damp or algae-stained patio.
Neither sealer replaces a wash. Film-forming acrylic sealers (a skin on the surface) are a third family again; if the SDS only names the solvent, you do not yet know the resin. Read the tin you hold.
Order of work
1. Identify the stain. 2. One wash chemistry, then rinse. 3. Dry. 4. Impregnate if water is soaking in. Degrease oil before you seal, or the sealer will cap the stain.
Never stack quat + bleach, acid + bleach, acid + alkali, or a solvent sealer into a bucket that still holds biocide. The area estimator and the teardown calculator turn square metres into litres once you have picked the molecule.
FAQ
- Should I seal a green patio?
- No. Water-repellents lock in whatever is on the stone. Kill or wash the growth first — BAC or DDAC for soft algae, hypochlorite or percarbonate where the stain needs an oxidiser — then let the masonry dry before a silane or siloxane.
- Is Lithofin Stainstop PLUS a drop-in for Thompson’s Water Seal?
- No. Thompson’s is water-based n-octyltriethoxysilane (CAS 2943-75-1). Stainstop PLUS is a solvent-borne dimethyl siloxane (CAS 69430-37-1) used neat, and it darkens absorbent stone. Same job family (impregnator), different molecule and different look.
- Will brick acid remove algae?
- Hydrochloric acid (Mortar Clen-style) shifts cement smears and mineral scale on acid-safe masonry. It is a poor algae killer and it etches limestone, marble and some sandstones. Soft green film wants a quat or an oxidiser, not brick acid.
More patio & exterior guides
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Patio cleaners decoded: BAC, DDAC, bleach, acid and degreasers
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How much patio chemical do you need? Measure first
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BAC vs DDAC vs bleach on patios — which active for which stain?
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When to use a metasilicate degreaser on driveways
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Driveway oil stains: degreaser vs solvent vs absorbent
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Driveway & TFR oil and grease: which solvent for which stain
Related teardowns
Live stacks this guide points at — mix maths and DIY affiliate links on each page.
Patio & exterior
Stainstop PLUS
Lithofin
- Retail pack
- £43.801 litre
- Raw active
- Lithofin Stainstop PLUS (same siloxane impregnator, 5 litre tin)69430-37-1
Not Thompson’s water-based silane. The GB SDS for Lithofin Stainstop PLUS lists an amino-functional dimethyl siloxane (CAS 69430-37-1) at 15–20% in hydrocarbon solvent, plus a trace of methanol. It sinks in and darkens absorbent stone. Pro Tiler Tools lists the same tin at £43.80 for 1 litre and £202.20 for 5 litres (inc VAT) — about £40 a litre in the big tin, an 8% tin-size gap, not a stronger molecule.
Patio & exterior
Water Seal Masonry Protector
Thompson's
- Retail pack
- £36.005 litres
- Raw active
- Water-based silane/siloxane masonry impregnator (50%)2943-75-1
A dilute siloxane in a 5 litre tin. Trade water-based silane/siloxane concentrates at 50% dilute 1+9 to about 5% working sealer — same job, more litres, less solvent perfume.
Patio & exterior
Wet & Forget Exterior Surface Cleaner
Wet & Forget
- Retail pack
- £34.005 litres
- Raw active
- Benzalkonium chloride (BAC 50)68424-85-1
A dilute quaternary ammonium biocide sold as a lifestyle patio product. The active is commodity BAC 50; the brand is paying for the trigger bottle and the television advert.
Patio & exterior
Patio Magic!
Patio Magic
- Retail pack
- £24.995 litres
- Raw active
- Benzalkonium chloride (BAC 50)68424-85-1
Another BAC patio biocide in a different coloured bottle. Same quaternary ammonium as Wet & Forget; same bulk drum of BAC 50. You are paying for the brand, not a different molecule.
Patio & exterior
4-in-1 Patio Power
Jeyes
- Retail pack
- £16.004 litres
- Raw active
- Didecyldimethylammonium chloride (DDAC 50)7173-51-5
This is not old Jeyes Fluid (tar acids are gone). Patio Power is 1.75% DDAC — a twin-chain quat, not the benzalkonium in Patio Magic. Dilute DDAC 50 to about 0.35% and you have the same working spray as the 4 L bottle mixed 1+4.
Patio & exterior
Hagesan Black Spot Remover
HG
- Retail pack
- £24.995 litres
- Raw active
- Sodium hypochlorite 14–15% (industrial / dairy grade)7681-52-9
Standard commercial bleach solution sold at five times agricultural merchant prices.
Patio & exterior
Eco Patio & Decking Cleaner
Logco
- Retail pack
- £29.995 kg tub
- Raw active
- Sodium percarbonate (technical oxygen bleach)15630-89-4
Logco’s Eco patio powder is sodium percarbonate (CAS 15630-89-4) — the same solid peroxide adduct as Vanish Oxi, sold as a 5 kg outdoor tub. Fifty grams per litre of warm water is the deck/patio band (about 5 m²/L). It is not BAC, not DDAC and not hypochlorite. A technical 5 kg sack is the same oxidiser without the decking label.
Patio & exterior
Mortar Clen Brick & Patio Acid
Everbuild
- Retail pack
- £16.505 litres
- Raw active
- Technical hydrochloric acid 32–36%7647-01-0
Retail brick acid is technical hydrochloric acid cut to about 9%. 5 litres of 36% acid plus 15 litres of water is 20 litres of the same working strength.
Patio & exterior
Organic Path, Patio & Deck Cleaner Concentrate
Algon
- Retail pack
- £11.992.5 litres
- Raw active
- Spirit vinegar 25% (acetic acid)64-19-7
The ‘organic patio cleaner’ is about 18% acetic acid with a drop of sugar-derived surfactant. Dilute 1:3 and you have ~4.5% vinegar. Spirit vinegar at 25% is the same molecule in a 5 litre jerry.
Patio & exterior
Patio, Driveway & Masonry Cleaner
KilrockPRO
- Retail pack
- £14.005 litres
- Raw active
- Sodium metasilicate pentahydrate10213-79-3
This is not Patio Magic (BAC) and it is not Mortar Clen (HCl). KilrockPRO is a dilute alkaline masonry degreaser — disodium metasilicate (CAS 6834-92-0) with soda ash and a chelator. Metasilicate pentahydrate crystals mixed at about 30 g/L match the bottle strength; then dilute 1:20 for light stains as the pack does.
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