Common stains found in commercial areas and how they are removed

Every commercial premises in London accumulates a specific combination of stains determined by how the building is used, who visits it, where it sits and how much traffic it receives. A retail forecourt in central London sees completely different contamination to a food service delivery yard or a car park beneath an office block. What remains constant across all commercial premises is the principle that correct stain identification must come before treatment selection. Applying an acid-based cleaner to organic biological growth, or an alkaline degreaser to efflorescence, will at best produce no result and at worst make the staining worse and damage the surface. The following guide covers the seven most common commercial stain types, what causes each one and how a professional contractor removes it correctly.

Oil, grease, diesel and fuel contamination

Where it appears: Car parks, delivery yards, petrol forecourts, loading bays, vehicle wash areas, garage floors and any surface where vehicles are parked or serviced.

Why it is difficult: Oil bonds to the porous surface of concrete and tarmac rapidly. The longer it is left, the deeper it penetrates the surface matrix, making later removal progressively more intensive and expensive. A fresh oil spill treated within 24 hours may require a single degreaser application. The same stain left for a week will require multiple applications and more aggressive chemistry.

Professional removal method: Apply a concentrated alkaline industrial Traffic Film Remover or dedicated oil degreaser to the affected area. The product must be specifically formulated for the surface type — degreasers designed for concrete can damage tarmac. Allow 10 to 20 minutes dwell time, agitate with a stiff brush to work the product into the pores of the surface, then pressure wash with hot water at the correct PSI for the surface. Hot water emulsifies the oil significantly more effectively than cold. For deeply ingrained historical oil contamination, a poultice application or multiple treatment cycles may be needed. Dispose of all wastewater via the foul drainage system, not surface drains.

Chewing gum deposits

Where it appears: Entrance areas, pedestrian footpaths, steps, car parks and any high-footfall paved surfaces. According to Keep Britain Tidy, approximately 87% of England's streets carry gum staining, making it one of the most prevalent commercial cleaning challenges in London.

Why it is difficult: Chewing gum bonds to paving at ambient temperatures, picking up surrounding debris and darkening over time into a black spot pattern that bears no visual resemblance to the original deposit. Cold-water pressure washing does not remove it effectively because it merely pushes the gum rather than breaking the chemical bond to the surface. Gum can take up to 5 years to biodegrade without treatment.

Professional removal method: Superheated steam or hot water at 150 degrees Celsius melts the gum and breaks the bond between the deposit and the surface without the need for chemicals or abrasive force. This is the preferred method on all paved surfaces and leaves no shadow staining or surface damage. For very high-density gum areas on entrance steps, a specialist gum remover chemical can be applied before steam treatment to accelerate the process. After treatment, any residue is pressure rinsed. Eco-blast systems using compressed air and fine abrasive are an alternative for delicate surfaces where heat cannot be applied.

Graffiti and paint contamination

Where it appears: Boundary walls, roller shutters, entrance facades, car park structures and any external surface accessible to foot traffic or accessible from a road.

Why it is difficult: Graffiti media vary considerably in composition. Aerosol spray paint, marker pen, paint applied with a brush, emulsion paint and bituminous compounds all penetrate surfaces to different depths and require different solvents to lift them. The faster graffiti is treated, the shallower the penetration and the easier the removal. Graffiti left on a porous surface for weeks allows the paint to cure fully and penetrate deep into the substrate.

Professional removal method: Apply a specialist graffiti remover gel formulated for the substrate and paint type. Gels are preferred to liquids because they cling to vertical surfaces and maintain contact dwell time without running off. After the dwell period, the softened paint is removed with low-pressure hot water or superheated steam. Multiple applications are standard for cured or aged graffiti. For porous brick and masonry, a second chemical application may follow the initial removal to clear residual pigment from the pores. Anti-graffiti protective coatings applied after removal create a sacrificial surface that makes future graffiti significantly easier to remove.

Bird guano and biological contamination

Where it appears: Ledges, canopies, flat roofs, loading bay roofs, car park structures and any elevated surfaces where pigeons or gulls roost. In London, pigeon roost sites on commercial buildings are extremely common.

Why it is difficult: Bird guano is highly acidic, with a pH of 3 to 4.5 comparable to vinegar. This acidity actively etches porous masonry, stonework, concrete and paintwork on prolonged contact. Heavy accumulations are also classified as hazardous waste under UK environmental regulations due to the presence of pathogens including Histoplasma capsulatum (Histoplasmosis), Cryptococcus neoformans (Cryptococcosis) and Salmonella bacteria. These organisms present genuine health risks to operatives and to building users. Businesses have a legal duty under the Health and Safety at Work Act 1974 and COSHH 2002 to ensure guano is managed safely.

Professional removal method: PPE including respirators, gloves and full protective clothing is mandatory. Dry guano is dampened first to prevent airborne particles. Bulk material is removed manually and bagged as hazardous waste. Remaining contamination is treated with an industrial biocidal disinfectant, pressure washed with hot water and the surface disinfected again. All waste is disposed of via licensed hazardous waste carriers. A professional contractor holds public liability insurance and the relevant COSHH assessments for this work. DIY removal of significant guano deposits is not advisable from a health and safety perspective.

Algae, moss and lichen on commercial surfaces

Where it appears: Car park floor surfaces, pedestrian paths, entrance steps, external brickwork and any surface in shade with regular moisture exposure. Biological growth creates significant slip hazards on commercial premises and is a frequent cause of public liability claims.

Why it is difficult: Pressure washing alone removes visible biological growth but leaves spores and root systems attached to the surface, allowing rapid recolonisation within weeks. For commercial premises where a cleaning visit happens once or twice a year, this is insufficient.

Professional removal method: For floors, a biocidal pre-treatment is applied and allowed to dwell for a minimum of 30 minutes, killing the growth at root level. The surface is then pressure washed at the appropriate PSI for the surface type. A post-treatment biocidal application after the clean significantly extends the period before regrowth, often doubling the interval between maintenance visits. For walls and facades, DOFF steam cleaning is preferred where applicable as it sanitises the surface and kills spores, providing a longer-lasting result than chemical treatment alone.

Rust streaks and iron staining

Where it appears: Below metal fixings, railings, overhead structures, steel-framed canopies and on concrete car park decks where reinforcing steel is close to the surface.

Why it is difficult: Rust staining on porous concrete and stone surfaces penetrates rapidly. Pressure washing and standard cleaners have no effect on iron oxide. Attempting to scrub rust staining abrasively often spreads it and increases the affected area.

Professional removal method: A chelating rust remover formulated for the specific surface type is applied and allowed to dwell for the time stated. These products chemically bind to the iron oxide particles and lift them from the surface pores. On concrete, oxalic acid-based treatments are effective on recent rust staining. On natural stone, a specialist non-acid rust remover must be used as acidic products will etch the stone permanently. After treatment, rinsing must be thorough. The source of the rust (the corroding metal element) must also be addressed to prevent recurrence.

Food grease, cooking oils and waste area contamination

Where it appears: Restaurant delivery areas, pub and cafe yard surfaces, food retail back-of-house areas, bin storage compounds and any surface near food preparation or service.

Why it is difficult: Animal fats and cooking oils from food service operations bond to concrete and paving more aggressively than petroleum products because they polymerise on contact with the surface as they cool and oxidise. Hot water pressure washing is significantly more effective than cold water on food grease because the heat breaks the bond between the fat and the surface. Food waste contamination also creates a persistent bacterial load that presents odour and hygiene issues beyond the visual staining.

Professional removal method: An enzymatic or alkaline industrial degreaser is applied hot where the machine allows, working into the surface with a brush before a hot water pressure wash using a surface cleaner attachment. For bin compounds and food waste areas, a follow-up biocidal disinfection step is included to address the bacterial load and eliminate odour-causing organisms. Businesses with food service operations typically require cleaning every 4 to 8 weeks to maintain safe, hygienic conditions and comply with environmental health requirements.