If black mildew or algae remains after pressure washing concrete, the problem is often not a lack of pressure. Biological growth and staining can remain in porous concrete even when loose surface dirt has been blasted away. Chemistry and dwell time may be the missing preparation step.
Why the Wand Can Disappoint
A pressure washer removes material by mechanical force. Increasing pressure can erode the cement paste or leave wand marks before it completely removes biological staining from the pores. A correctly sized surface cleaner improves uniformity, but it is still a mechanical tool; it does not replace a cleaner formulated to kill or loosen mildew.
A Better Sequence Before Coating
- Protect and pre-wet nearby vegetation as the cleaner label directs.
- Apply a concrete/exterior mildew cleaner that is compatible with the substrate and the later coating process.
- Give the product its labeled dwell time; do not immediately wash it away.
- Agitate stubborn areas with an appropriate nonmetal brush if the label allows it.
- Pressure wash or surface-clean the concrete evenly.
- Rinse thoroughly enough that no cleaner residue remains to interfere with the coating.
- Allow the slab to dry and then continue with whatever profiling the coating system requires.
Sodium Hypochlorite Products
Some driveway and pressure-washer cleaners use sodium hypochlorite as the active ingredient. Mold Armor’s current pressure-washer concentrate label, for example, lists 5.25% sodium hypochlorite and is labeled for mold/mildew on concrete driveways. Follow the product label for dilution, PPE, plant protection, dwell and rinsing.
Do Not Create a Chemical Hazard
Do not mix chlorine-bleach/sodium-hypochlorite cleaners with acids, ammonia or other cleaners. If an acid etch is part of a later coating-preparation step, completely rinse the bleach cleaner away and treat the operations as separate processes.