· by Michael Blair, Nashville's Pressure Washing
Concrete absorbs. A stain has usually penetrated below the surface, which is why pressure washing often lightens it and does not remove it. Each type needs a different chemistry.
Oil and grease
A degreaser with dwell time, agitated with a stiff brush, then rinsed. For older set-in stains, a poultice — an absorbent material mixed with solvent, applied thickly and covered — draws the oil back out as it dries.
That takes a day or two and works far better than any amount of pressure.
Rust
Rust needs an acid-based remover formulated for it, usually oxalic acid. General cleaners do nothing.
It is common in Middle Tennessee from irrigation water with iron in it, and from metal furniture left standing.
- Oil: degreaser and dwell, then a poultice for old stains
- Rust: oxalic acid based remover
- Organic and leaf tannin: sodium hypochlorite solution
- Efflorescence: brush dry, address the moisture
- Always test in an inconspicuous area
Organic staining
Leaf tannin, algae and mildew respond to a dilute sodium hypochlorite solution with dwell time. This is the most common driveway discoloration here under trees.
It is also the easiest to remove and the one people most often attack with unnecessary pressure.
Sealing afterwards
A penetrating sealer reduces future absorption considerably and makes the next clean easier. It is not a film, so it does not change appearance much or peel.
Apply to clean dry concrete, and expect to reapply every few years.