Pressure Washing a Commercial Building Before Painting: How to Clean It Without Damaging It
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To clean a commercial building before painting without damaging it, match the method to the wall. Sound brick, block, and tilt-up concrete can usually take careful pressure washing. EIFS, stucco, and older soft masonry call for a soft wash with detergent and mildewcide. Either way, test a small area, control the runoff, and let the walls dry completely.
Cleaning is the least glamorous part of a commercial exterior job and the one that decides whether the new finish sticks. We see it on buildings all over St. Louis and St. Charles County: a fresh coat of paint peeling in sheets a couple of years later, with a layer of chalk or mildew still stuck to the back of every flake. The wall was never really clean.
Why does cleaning matter so much before coating?
Coatings bond to whatever is on the surface. If that's dirt, chalk, mildew, or loose old paint, the new film is only as strong as that layer.
Missouri makes this harder than it sounds. Humid summers grow mildew fast on shaded walls and anywhere landscaping holds moisture against the building. Old paint on south and west elevations breaks down under UV and turns chalky. Exhaust and road grime build up near docks and parking lots. All of it has to come off before primer goes on, and the cleaning method has to remove it without gouging mortar, forcing water into the wall, or etching the surface.
What's the difference between pressure washing and soft washing?
Pressure washing uses water force to strip dirt and loose material. Soft washing uses low pressure and relies on cleaning solutions to do the work, followed by a thorough rinse.
Neither one is "better." They solve different problems. High pressure on the wrong surface can cut into soft brick, blow out old mortar, drive water behind siding or into EIFS, and raise fibers on wood. Low pressure alone won't lift heavy chalk from aging metal panels. The right call depends on the substrate, its condition, and what you're trying to remove. Pressure settings vary by substrate, so a good crew starts low and tests a small, out-of-the-way patch before working a whole wall.
How should each type of commercial wall be cleaned?
Each substrate has its own risks. Here's how we generally approach the walls we see most on commercial buildings around the metro.
| Substrate | Usual approach | What to watch for |
|---|---|---|
| Brick | Careful pressure washing on sound brick; soft wash on older or soft brick | Eroded mortar joints, spalling faces, white efflorescence that points to moisture |
| Concrete block (CMU) | Pressure washing at moderate settings | Very porous; soaks up water and needs extra drying time |
| EIFS | Soft wash only | Never blast it like brick; have it inspected for moisture problems first |
| Stucco | Soft wash or gentle low-pressure rinse | Hairline cracks and loose patches that high pressure can open up |
| Metal panels | Pressure wash plus detergent to remove chalk | Rust at fasteners and laps, failed panel sealant, water forced into seams |
| Tilt-up concrete | Pressure washing on sound panels | Panel joint sealant, surface residues, and any cracks that need repair before coating |
EIFS deserves its own warning. It looks a lot like stucco, but it's a foam-based system that can hide water damage behind a sound-looking face. Before anyone washes an EIFS wall, it should be inspected for moisture problems behind it, and any damage needs to be handled first. If your walls turn out to be traditional stucco instead, our page on stucco coatings covers how we prep and protect them.
What detergents and mildewcides are used?
Most commercial cleaning combines a detergent for dirt and chalk with a mildewcide for organic growth. The specific products depend on the wall and what's growing on it.
Mildewcide matters more than people expect in a Missouri summer. If you only wash mildew off and don't kill it, it's back on the north side before the job is finished. Equally important is the rinse. Cleaning residue left on the wall is just another layer between the substrate and the primer, so every treated surface gets rinsed thoroughly. Plants, lawn edges, and anything the crew can't move get wetted down or covered before chemicals go on.
How do you handle runoff, tenants, and parking?
Plan it before the first wall gets wet. Runoff, overspray mist, and wet walkways all affect people who are trying to run a business.
- Runoff. Cover or protect storm drains and landscaping, and follow whatever local rules apply to wash water at your site.
- Parking. Cars near the active wall should be moved. Wash mist travels farther than you'd think on a breezy day.
- Entrances. Keep at least one entrance dry and open, with signage pointing customers to it.
- Windows and HVAC. Tenants on the active side should keep windows closed. Rooftop or wall-mounted air intakes may need to be shut down or shielded briefly.
- Notice. Give tenants written dates for each elevation and one contact for questions.
A clean, organized job site is what owners tend to remember afterward. One commercial customer wrote this after we finished his building:
"Cannot thank Darrin and his team enough for their amazing work. They were easy to deal with, fast, and clean with their work. Our building looks amazing!"
— Tanner B.
"Clean with their work" starts on wash day. That's when tenants decide whether the project is a minor hassle or a real one.
How long should walls dry before coating?
Long enough to be dry all the way through. Dry to the touch doesn't count. That's usually at least a day, and longer for porous surfaces.
Concrete block and older brick hold a lot of water, and a humid June week in St. Louis slows drying down noticeably. Coating a wall that's still damp inside traps moisture under the film, which leads to blistering and poor adhesion later. Shaded elevations dry slower than sunny ones, so a building often dries unevenly. A good crew checks each wall before priming rather than starting on a calendar date. If a storm rolls through after washing, the clock resets.
At Rhino Shield of Missouri, cleaning is the first step of a multi-step system: clean, repair, caulk and seal, prime, then a multi-coat ceramic topcoat applied at 8 to 10 mils. You can read the full sequence on our application process page. Trade publications like CoatingsPro Magazine cover surface prep for commercial coatings in depth, and the theme is always the same: prep decides service life. Done right, the finish keeps its look for decades instead of needing a redo in 5 to 7 years. Our commercial exterior painters in St. Louis build every bid around that prep.
Frequently Asked Questions
Can you pressure wash EIFS?
Not the way you'd wash brick. EIFS should be soft washed at low pressure, because high pressure can damage the finish and force water behind the system. It should also be inspected for moisture problems before any cleaning or coating.
What pressure should be used to wash a commercial building?
There's no single number. The right setting depends on the substrate, its age, and its condition, so sound tilt-up concrete can take more than soft older brick or stucco. A good crew starts low and tests a small area first.
Does pressure washing kill mildew on a building?
Not by itself. Water pressure removes visible growth, but a mildewcide is needed to kill what's left in the surface. Without it, mildew can return quickly on shaded walls in a humid Missouri summer.
How long after washing can you paint a commercial building?
Usually at least a day, and longer for block, older brick, and shaded walls. The wall needs to be dry through its depth, and a dry-looking surface can still hold water underneath. Rain after washing adds more drying time.
Get Your Building Assessed Before Wash Day
A good cleaning plan starts with knowing what your walls are made of and what shape they're in. Request a commercial estimate and we'll walk every elevation with you, or call 314-239-7947 Monday through Friday, 8 a.m. to 6 p.m.
