Professional exterior cleaning is not just water and pressure. The chemicals used alongside mechanical cleaning are often what make the difference between a surface that looks good and one that actually gets clean at the biological level. This guide covers every major chemical category used in the professional pressure washing industry: what each compound is, what it does, which surfaces it is appropriate for, and what homeowners and property owners should know about safety and environmental considerations.
Sodium Hypochlorite (SH): The Foundation of Soft Washing
Sodium hypochlorite is the active ingredient in household bleach, but professional-grade SH is significantly more concentrated. The bleach you buy at the grocery store is typically 3-6% sodium hypochlorite. Professional cleaning-grade SH is commonly available at 10-12.5% concentration and is the foundation chemical for soft washing applications.
What it does: Sodium hypochlorite is a strong oxidizing agent. It kills biological organisms — algae, mold, mildew, lichen, bacteria — by disrupting their cellular metabolism and oxidizing the pigments they produce. It is effective against virtually all biological growth found on exterior surfaces.
When it is used: Roof cleaning, house washing (soft wash), fence cleaning, and anywhere biological growth (algae, mold, mildew) is the primary contamination. SH is not appropriate for raw metal surfaces, unpainted galvanized metal, certain natural stones, or surfaces that have been treated with incompatible coatings.
Application concentrations: The appropriate working concentration varies significantly by surface:
- Asphalt shingle roofs: 3-6% (mixed from 10% stock)
- Vinyl siding: 0.5-2%
- Wood siding: 1-3% (higher concentrations can damage wood fibers)
- Concrete with heavy biological growth: 1-3%
Safety considerations: SH is corrosive at high concentrations. It causes skin and eye irritation, can bleach fabric, and kills vegetation on contact. Professional applicators wear appropriate PPE and pre-wet surrounding plants before application, rinse plants during and after application, and avoid application on windy days that cause drift onto non-target surfaces.
Environmental considerations: Sodium hypochlorite breaks down relatively quickly in the environment. It reacts with organic material (the very material it is cleaning) and neutralizes. Properly applied SH solutions that are adequately rinsed after treatment pose minimal long-term environmental risk. However, discharge of high-concentration SH solutions directly into waterways or storm drains without dilution is problematic and should be avoided.
Surfactants: Making Cleaning Chemistry Work
A surfactant (surface-active agent) is a compound that reduces the surface tension of water and helps cleaning solutions penetrate, spread, and lift contamination. Every professional cleaning mix includes a surfactant component.
How they work: Surfactant molecules have a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. The hydrophobic tail attaches to grease, oil, or biological material on the surface. The hydrophilic head stays oriented toward the water phase. When you rinse, the surfactant molecule carries the contaminant away with the water — this is the mechanism of all soap and detergent cleaning.
In soft wash applications: Surfactants are mixed with SH to create a stable cleaning solution that clings to vertical surfaces (dwell time) rather than immediately running off. Without a surfactant, SH solution applied to a wall would run off within seconds, not providing adequate contact time to kill organisms. The surfactant holds the solution in contact with the surface during the dwell period.
Types used: Nonionic surfactants are the most common in exterior cleaning because they are compatible with SH, effective across a wide pH range, and biodegradable. Anionic surfactants (like sodium lauryl sulfate, found in shampoos) are also used in some formulations. Cationic surfactants have antimicrobial properties of their own and may be used in specialized applications.
Environmental considerations: Modern surfactants used by reputable exterior cleaning contractors are biodegradable and break down in the environment within days to weeks. Avoid contractors using petroleum-based surfactants or industrial degreasers that are not labeled as biodegradable — these can persist in soil and groundwater.
Degreasers: Tackling Petroleum Contamination
Oil, grease, and petroleum contamination requires a degreaser — a chemically alkaline compound that breaks down hydrocarbon chains through a process called saponification (converting fats into soap-like compounds that can be rinsed away).
Common active ingredients: Sodium hydroxide (caustic soda), potassium hydroxide, and various proprietary alkaline builders are common in commercial degreasers. Citrus-based degreasers use d-limonene (a terpene derived from citrus peel) as the active degreasing agent and are preferred for applications near landscaping or where biodegradability is a priority.
When degreasers are used:
- Motor oil stains on driveways and parking lots
- Cooking grease on restaurant exteriors and loading dock areas
- Hydraulic fluid or machine oil on industrial surfaces
- Tire rubber marks on parking lot concrete
- Fuel spills on gas station concrete
Application method: Degreasers are typically applied at full or moderate concentration as a pre-treatment, allowed to dwell for 5-15 minutes to emulsify the petroleum contamination, agitated with a brush or low-pressure spray, and then pressure washed away. Hot water significantly improves degreaser performance — the heat accelerates saponification and helps the emulsified grease flow freely from the surface.
Safety: High-concentration sodium hydroxide solutions are caustic and can cause severe skin and eye burns. Professional applicators use full PPE and take care around aluminum surfaces, which react with sodium hydroxide to produce hydrogen gas.
Oxalic Acid: The Rust and Tannin Remover
Oxalic acid is a naturally occurring organic acid found in many plants (spinach, rhubarb) that has powerful chelating properties — it bonds to metal ions and holds them in solution, removing mineral stains from surfaces.
Applications:
- Rust stains from irrigation systems, fertilizer, and metal objects on concrete
- Tannin staining from oak trees, leaves, and wood on concrete and stone
- Iron staining on brick and masonry
- Wood brightening after deck cleaning (restores natural wood color by removing oxidized wood cells and iron-based gray coloration)
See our detailed guide on removing rust stains from concrete for specific application instructions.
Safety: Oxalic acid is toxic if ingested and a moderate skin/eye irritant. It is harmful to most plants. Proper rinsing and neutralization after application is important. Do not mix with bleach or oxidizing agents.
Sodium Hydroxide: Heavy-Duty Cleaning
Sodium hydroxide (NaOH, also called caustic soda or lye) is a strongly alkaline compound used as the active ingredient in heavy-duty degreasers and in some specialized cleaning applications.
Used for: Industrial degreasing, heavy carbon deposit removal on BBQ grates and cooking equipment exteriors, stripping certain types of paint or coatings, and cleaning heavily contaminated concrete in commercial and industrial settings.
Limitations: Sodium hydroxide is extremely corrosive and must be handled with full PPE. It reacts aggressively with aluminum, zinc, and certain other metals. It is not appropriate for general residential use and should only be applied by experienced professionals who understand neutralization requirements. It should never be mixed with acids (including oxalic acid) as mixing causes violent reactions.
Phosphoric Acid: Efflorescence and Mineral Deposit Removal
Phosphoric acid is used to remove efflorescence (the white salt deposits that appear on brick, concrete, and stucco as minerals leach from within the substrate), calcium deposits, and hard water scale from exterior surfaces.
How it works: Phosphoric acid reacts with calcium carbonate (the primary component of efflorescence and hard water deposits) to form water-soluble calcium phosphate, which can then be rinsed away. It is gentler than muriatic acid and leaves a phosphate coating on metal surfaces that temporarily inhibits corrosion.
Applications in exterior cleaning: Post-construction cleaning of brick and masonry, removing efflorescence from retaining walls and concrete block, treating hard water deposits on glass and tiles near water features.
What "Eco-Friendly" Means in Exterior Cleaning
The term "eco-friendly" is used loosely in the industry. Here is what it actually means in practice:
Truly environmentally responsible exterior cleaning means using biodegradable surfactants, properly diluted SH (which degrades quickly), controlling runoff to prevent concentrated chemicals from reaching storm drains or waterways, and not using persistent chemical compounds in residential or near-waterway applications.
Responsible contractors will pre-wet vegetation, rinse plant material before and after chemical application, and manage runoff on sites near natural water features. They will also select the least aggressive chemistry that achieves the required result — not defaulting to maximum concentration when diluted solutions will suffice.
At Thrare Contracting, we select chemicals specifically appropriate to each surface and contamination type, use biodegradable surfactants, follow all applicable runoff guidelines, and take particular care on properties in the Stone Mountain area near Arabia Mountain and Davidson-Arabia Mountain Nature Preserve. Questions about our chemical practices? Call us at (678) 748-3578.