Surface stain or deep contamination
Concrete is porous. A drip that landed last week sits in the top fraction of a millimetre and lifts easily. A drip that has been landing in the same spot for six years has wicked down through the paste and around the aggregate, and no amount of surface work reaches it.
Test with water. Wet the stain: if it darkens uniformly with the surrounding concrete, the slab is still absorbing there and the contamination is shallow. If the stain repels water and stays lighter while everything round it darkens, the pores are full of oil and you are into poultice territory — or an honest conversation.
| Age and depth | Realistic outcome | Method |
|---|---|---|
| Days to weeks | Gone | Degreaser, dwell, agitate, rinse |
| Months | Mostly gone, faint ghost | Degreaser twice, then hot water |
| Years, repeated | Lightened, not removed | Poultice, possibly repeated |
| Transmission or hydraulic fluid | Often permanent | Poultice; tell them first |
Degreaser and dwell
Apply an alkaline degreaser to the dry stain — wetting first dilutes it before it reaches the oil. Give it the dwell the label asks for, which is usually longer than anyone waits. Agitate with a stiff brush to break the surface tension of the film, then rinse at working pressure.
Then look again while it is wet, and again when it is dry. Concrete lies while wet: everything looks uniform. Judge the result dry, and expect to repeat the cycle once on anything more than a few months old.
Poulticing a deep stain
A poultice is an absorbent carrier — diatomaceous earth, cat litter ground fine, or a proprietary powder — mixed with a solvent into a paste, spread over the stain, covered, and left for a day or two. As the solvent evaporates it draws the oil up out of the pores and into the powder, which you then sweep away.
- Spread it thicker than feels necessary — a quarter of an inch at least, extending past the visible stain.
- Cover with plastic taped at the edges, so it dries slowly from the centre out.
- Leave 24–48 hours. Rushing it is the most common failure.
- Expect to repeat. Two or three cycles on a bad stain is normal, not a sign it failed.
Poultice is a return visit, and it is billable
What heat adds
A hot-water machine changes the arithmetic on petroleum specifically. Heat lowers the viscosity of the oil and helps the surfactant get underneath it, so a stain that takes three cold cycles may take one hot one. That is the strongest single argument for owning a hot-water machine, and it is why fleet and restaurant work is dominated by operators who have one.
It is not magic. Heat helps petroleum and organic fats. It does nothing for mineral staining, nothing for rust, and nothing for a stain that has already penetrated deeper than the water reaches.
PSI & nozzle quick reference
- 0 degRed
- Spot jet. Etches most surfaces.4000 PSI
- 15 degYellow
- Stripping bare concrete.4000 PSI
- 25 degGreen
- General concrete and walkways.3500 PSI
- 40 degWhite
- Siding, wood, vehicles.1500 PSI
- ~65 degBlack
- Detergent draw, low pressure.500 PSI
Setting expectations before you start
The single most useful sentence in this trade: “This will lighten considerably. I do not expect it to disappear, and here is why.” Said at the quote it is expertise. Said at the invoice it is an excuse.
- Photograph the stain before you touch it, from the angle the customer looks at it.
- Name it on the estimate as its own line, with its own price and its own expected outcome.
- Show them the wet test and explain what it means.
- Offer the poultice as a follow-up rather than promising it inside the wash.
Sometimes the honest answer is a coating