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Wet British sandstone patio with algae in the joints

Category hub

Patio & exterior

Algae, lichen and patio films — same quaternary biocide, without the brand tax.

Teardowns

8

Typical saving

76%

Steepest gap

Wet & Forget Exterior Surface Cleaner

How to buy in this category

Patio films, black spot and rust stains are almost never unique molecules. The bottle is a dilute biocide, bleach or oxalic solution with a trigger and a television advert.

Buy the concentrate, mix to the labelled working strength, and treat on a still, dry day. Rinse nearby plants and keep pets off until the surface is dry.

If two brands share a CAS number, pick the teardown with the mixing maths that matches the job — algae film versus iron stain — not the prettier pack.

This hub currently covers 8 patio & exterior teardowns. Typical saving versus a branded pack is about 76% per working litre. The steepest gap today is Wet & Forget Exterior Surface Cleaner at 88%. Snapshot retail prices start around £8.50 — live shop listings move.

Compare actives

Wet British sandstone patio with algae in the joints
Save 88%

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.

Gloves. Chemical-resistant gloves (nitrile or neoprene)Eye protection. Indirect-vent splash goggles — not ordinary spectaclesVentilation. Use outdoors or with mechanical extraction
Open teardownBranded pack
Wet British sandstone patio with algae in the joints
Save 84%

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.

Gloves. Chemical-resistant gloves (nitrile or neoprene)Eye protection. Indirect-vent splash goggles — not ordinary spectaclesVentilation. Use outdoors or with mechanical extraction
Open teardownBranded pack
Wet British sandstone patio with algae in the joints
Save 86%

Patio & exterior

HG Rust Stain Remover

HG

Retail pack
£8.50500 millilitres
Raw active
Oxalic acid dihydrate6153-56-6

Oxalic acid is the classic rust and tannin bleach for masonry. HG bottles a dilute solution; a kilogram of crystals makes tens of litres of the same chemistry.

Gloves. Chemical-resistant gloves (nitrile or neoprene)Eye protection. Indirect-vent splash goggles — not ordinary spectaclesVentilation. Use outdoors or with mechanical extraction
Open teardownBranded pack
Wet British sandstone patio with algae in the joints
Save 64%

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.

Gloves. Chemical-resistant gloves (nitrile or neoprene)Eye protection. Indirect-vent splash goggles — not ordinary spectaclesRespirator. Rated respirator for dusts or vapours as specified on the SDSVentilation. Use outdoors or with mechanical extraction
Open teardownBranded pack
Wet British sandstone patio with algae in the joints
Save 73%

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.

Gloves. Chemical-resistant gloves (nitrile or neoprene)Eye protection. Indirect-vent splash goggles — not ordinary spectaclesRespirator. Rated respirator for dusts or vapours as specified on the SDSAcid-resistant PPE. Apron and gloves rated for corrosive liquids
Open teardownBranded pack
Wet British sandstone patio with algae in the joints
Save 64%

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.

Gloves. Chemical-resistant gloves (nitrile or neoprene)Eye protection. Indirect-vent splash goggles — not ordinary spectaclesVentilation. Use outdoors or with mechanical extraction
Open teardownBranded pack
Wet British sandstone patio with algae in the joints
Save 70%

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.

Gloves. Chemical-resistant gloves (nitrile or neoprene)Ventilation. Use outdoors or with mechanical extraction
Open teardownBranded pack
Wet British sandstone patio with algae in the joints
Save 78%

Patio & exterior

Rust Remover Gel

Jenolite

Retail pack
£8.99500 millilitres
Raw active
Phosphoric acid 81% (food / technical) diluted to ~25%7664-38-2

This is not oxalic (that is HG masonry rust) and it is not hydrochloric (Mortar Clen). Phosphoric acid etches rust to iron phosphate. Technical 81% phosphoric diluted to about 25% is the same attack without the gel tax. Keep it off limestone.

Gloves. Chemical-resistant gloves (nitrile or neoprene)Eye protection. Indirect-vent splash goggles — not ordinary spectaclesVentilation. Use outdoors or with mechanical extractionAcid-resistant PPE. Apron and gloves rated for corrosive liquids
Open teardownBranded pack