The number one reason pressure washing jobs fail to clean properly isn't the equipment -- it's using the wrong chemical for the stain type. Blast rust with bleach and you'll waste 30 minutes and accomplish nothing. Apply the right oxalic acid solution and it's gone in 10. Here's how to identify what you're dealing with and exactly what to do about it.

The Quick Answer

Different stains need different treatments. Standard pressure washing only removes loose dirt and surface organic growth. Embedded stains -- rust, oil, tannins, mineral deposits -- all need a specific pre-treatment chemical before the water ever touches them.

Rust (orange/red streaks): Oxalic acid or citric acid solution
Mold and mildew (black/dark fuzzy patches): Sodium hypochlorite 1-3% + surfactant, soft wash
Algae (green slick film): Sodium hypochlorite + surfactant, downstream injection
Tannin stains (brown ghost marks): Oxalic acid on wood, alkaline cleaner on concrete
Oil and grease (dark wet-looking patches): Heavy-duty degreaser + 2,500+ PSI
Mineral/hard water deposits (white crusty buildup): Acidic descaler or phosphoric acid

Read on for step-by-step treatment for each stain type -- and how to price specialty removal so you're not doing harder work for the same rate.

Identifying and Removing Rust Stains

Rust shows up as orange or reddish-brown streaks, usually running down from metal furniture legs, irrigation heads, or iron pipes. You'll see it most often on driveways, sidewalks, and concrete patios near sprinkler systems.

Never use bleach on rust. Sodium hypochlorite does nothing to iron oxide and can actually set the stain deeper into the concrete.

Treatment: Apply an oxalic acid-based rust remover or citric acid solution directly to the stained area. Let it dwell 10-15 minutes -- you'll see the orange color lift and fade. Then rinse at 2,500-3,000 PSI. For old or deeply embedded stains, apply 1-3 coats before rinsing.

Pricing: Standard concrete cleaning runs $0.10-0.25/sq ft. Rust removal is specialty work -- charge $0.60-$3.00/sq ft depending on severity, or add a flat chemical treatment fee of $75-150 on top of your base quote.

Identifying and Removing Mold and Mildew

Mold looks black, dark gray, or greenish-black and has a fuzzy or slimy texture. It grows in shaded, damp areas -- north-facing walls, under overhangs, underneath wood decks. Mildew is the lighter, powdery version, usually appearing white or gray.

High-pressure washing alone won't kill mold. You'll blast it off and it's back in 30-60 days because the spores are still alive underneath the surface.

Treatment: Apply sodium hypochlorite at 1-3% concentration mixed with a surfactant. Use soft wash technique -- low pressure, downstream injection -- to coat the surface evenly. Let it dwell 5-10 minutes. The mold will die and turn brown. Then rinse at 1,000-1,500 PSI.

This is why soft washing beats high pressure on mold. You're treating the biology, not just blasting the surface appearance.

Identifying and Removing Algae

Algae is green, slimy, and grows in sunlit wet areas -- the opposite of where mold thrives. You'll see it on driveways near sprinkler systems, on roofs (those dark black streaks are actually a type of algae called Gloeocapsa magma), and on concrete near trees or water features.

Treatment: Same approach as mold -- sodium hypochlorite plus surfactant, soft wash application, 5-10 minute dwell, then rinse at low to moderate pressure. High pressure alone removes algae temporarily but doesn't kill the root structure. It's back in weeks.

On roofs, never use high pressure. Apply soft wash solution, let it dwell 15-20 minutes, then rinse at low pressure. Here's a full guide on how to soft wash a roof safely.

Identifying and Removing Tannin Stains

Tannin stains are the brown ghost marks left behind after wet leaves sit on concrete, wood decking, or composite surfaces for too long. They look like faint brown shadows in the exact shape of leaves or organic matter -- and they're common on decks in fall and driveways near large trees.

These stains are often underestimated. They can't be pressure washed off with water alone.

Treatment: On wood, oxalic acid works well -- apply, dwell 10 minutes, scrub lightly, then rinse. On concrete, try an alkaline cleaner first. Apply, scrub lightly, dwell 5-10 minutes, then high-pressure rinse. Stubborn stains may need 2 treatments. Sun exposure fades what little remains.

Identifying and Removing Oil and Grease Stains

Oil stains look like dark wet patches on concrete even when the surface is completely dry. Most common on garage floors, driveways, and commercial parking lots. Fresh oil stains under 30 days old respond much better to treatment than set-in stains that have been sitting for months.

Treatment (step by step):

For fresh stains, spread kitty litter over the spot first to absorb excess oil. Leave 24 hours, then sweep before applying any chemical.
Apply a heavy-duty industrial degreaser and let it soak 15-30 minutes.
Pressure wash at 2,500+ PSI with at least 2.5 GPM. You need flow, not just pressure, to flush the broken-down oil out of the pores.
Rinse and inspect. Old, set-in stains usually need a second pass.

Pricing: Oil stain removal is a high-margin specialty service. Standard concrete cleaning doesn't touch embedded oil -- charge accordingly. Most contractors price oil stain jobs at 3-5x their standard concrete rate. See our guide on removing oil stains from concrete for a full breakdown.

Identifying and Removing Hard Water and Mineral Deposits

Hard water deposits appear as white or gray crusty buildup on surfaces that get regular water exposure. Common around pool decks, on walls near sprinkler heads, on glass, and on tile.

Pressure washing alone won't remove mineral deposits. You need acid to dissolve calcium and mineral buildup -- water just slides off the surface.

Treatment: Apply an acidic descaler or phosphoric acid solution to the affected area. Let it dwell 5-15 minutes. You'll see the mineral buildup begin to fizz as it dissolves. Rinse with moderate pressure. Never use muriatic acid on colored or sealed concrete -- it strips the finish.

How to Quote Specialty Stain Removal Jobs

Every specialty stain type costs more because of the chemicals and dwell time involved. Here's how to structure your quote so you don't accidentally bury specialty work in your base rate:

Base service: Standard pressure washing at your normal per-sq-ft rate for the surface type
Specialty stain treatment: Flat fee ($75-200) for chemical application, OR a premium per-sq-ft rate for the stained area ($0.60-$3.00 depending on stain type and severity)
Multiple stain types: Charge per treatment type. Rust + oil + mold is three separate chemical treatments -- quote them separately.

Always inspect the surface before quoting. Walk the driveway, check the walls, look at the surface condition. Never bundle specialty stain treatment into your base rate. It's extra work, extra chemicals, and extra time -- and it's completely justified as a line item.

Bottom Line

Identify the stain type before you pick up the pressure gun. The right chemical makes the difference between a 10-minute treatment and an hour of scrubbing that still doesn't work. And when you price specialty stain jobs correctly, they become some of the most profitable work you'll run all week.

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How to Identify and Remove Pressure Washing Stains (Complete Guide 2026)