TL;DR
Oil sits inside the pores of concrete, not on top of it, so pressure alone will never remove an old stain -- it only cleans the surface above the oil. The working sequence is absorb what is still loose, degrease with real dwell time, then use a poultice that pulls the remaining oil back out as it dries. Old stains often take several rounds.
Why pressure does not work on oil
Concrete is porous. Motor oil, transmission fluid, and cooking grease are liquids that wick down into that pore structure, sometimes several millimetres deep, and they stay there.
A pressure washer cleans the surface. The oil below the surface is untouched, and within days it migrates back up and the stain reappears looking almost as it did before.
Hot water helps a little because it lowers the oil viscosity. Chemistry helps a lot, because a degreaser emulsifies the oil so water can carry it. But the real problem is depth, and depth needs something that pulls upward.
That is what a poultice does.
Source: concrete restoration and stain removal practice guidance
Act fast on a fresh spill
Everything is easier in the first hour.
Do not hose it. Water spreads a fresh spill and drives it deeper.
Absorb it. Cover the spill generously with a dry absorbent -- clay-based cat litter, sawdust, cornstarch, or a commercial oil absorbent. Let it sit for at least a few hours, ideally overnight, then sweep it up and dispose of it appropriately.
Repeat if the absorbent turns dark quickly. A second application on a heavy spill is normal. Then degrease what remains, following the steps below.
Degreasing the stain
For a stain that is days to weeks old, or after absorbing a fresh spill:
1. Apply an alkaline degreaser rated for concrete, at the label dilution. Alkaline chemistry breaks oil into something water can carry away.
2. Let it dwell. Ten to fifteen minutes is common, and longer for heavy staining. Do not let it dry -- mist with plain water if it starts to.
3. Agitate. A stiff-bristle brush or a push broom, worked into the surface in both directions. Agitation matters as much as the chemistry.
4. Rinse thoroughly and look at the stain while the slab is still wet. Wet concrete hides a lot, so judge the result only after it dries.
5. Repeat if the stain lightened but did not clear. Two or three cycles of degreaser and agitation will handle a great many stains.
Do this before you wash the rest of the slab, not after, so the finished driveway comes up evenly. That sequencing is the point in how to clean a concrete driveway.
The poultice method for old stains
A poultice is an absorbent material mixed with a solvent into a paste. Spread over a stain and left to dry, the solvent dissolves the oil deep in the pores and the drying paste draws it upward into itself.
What to use. An absorbent powder -- diatomaceous earth, clay-based cat litter ground fine, powdered chalk, or a commercial poultice powder -- mixed with a solvent appropriate to the stain. For oil, a degreaser solution or an approved solvent works.
How to do it:
- Mix the powder and solvent into a paste roughly the consistency of thick peanut butter.
- Spread it over the stain about a quarter inch thick, extending an inch or two past the visible edge.
- Cover with plastic sheeting taped at the edges and leave for a few hours, which keeps the solvent working rather than flashing off.
- Remove the plastic and let the poultice dry completely, usually a full day or longer. This drying stage is when the oil is actually drawn out.
- Scrape up the dry poultice and dispose of it appropriately.
- Rinse and assess once the concrete is dry.
Repeat as needed. A years-old stain may need three or four cycles.
Solvent safety matters here. Work outdoors with air moving, keep solvents away from flames, sparks, pilot lights, and running engines, wear gloves and eye protection, and follow the product label. Never mix solvents together, and never combine any solvent or degreaser with a bleach-based product.
Other approaches, and what they are worth
Microbial or enzyme-based oil removers use organisms that digest hydrocarbons. They are slow -- days to weeks -- and need the surface to stay damp, but they are effective on old stains and low-hazard.
Baking soda or cornstarch pastes work as a mild poultice on light, recent stains. Dish soap and a brush helps a fresh spill and does nothing for a soaked-in one.
Acidic etching products remove the stain by removing the stained surface layer of concrete, leaving a lighter, rougher patch that stands out. Last resort only.
Never combine an acid with a bleach-based cleaner. That releases chlorine gas. If you have used one product, rinse the slab until the runoff is clear before another touches it.
Managing expectations
Some stains do not come out completely.
Oil that has been in place for years, in a slab that has never been sealed, may have gone deep enough that the discoloration is effectively part of the concrete. Repeated poultice cycles will keep lightening it, with diminishing returns.
At that point the options are acceptance, a coating that changes the whole slab colour, or resurfacing. Spot-fixing does not blend, for the same reason a re-cleaned band stands out -- see avoiding stripes and lap marks on concrete.
Preventing the next one
Fix the leak. Everything else is treating a symptom. Put a drip tray under a vehicle known to leak, and change it before it saturates.
Seal the concrete. A penetrating sealer fills the pores so future spills sit on top where they can be wiped up rather than soaking in. Concrete must be completely dry before sealing, which usually means several days after any washing.
Keep an absorbent on hand so a spill can be covered immediately. Garage-floor buildup is its own project, covered in degreasing a garage floor with years of buildup.
FAQ
How do you remove old oil stains from concrete?
Apply an alkaline degreaser with ten to fifteen minutes of dwell time and stiff-brush agitation, repeating two or three times. For stains that remain, use a poultice -- an absorbent powder mixed with solvent into a paste, spread over the stain and allowed to dry, which draws oil up out of the pores.
Will pressure washing remove oil stains from a driveway?
No. Oil sits inside the pore structure of the concrete rather than on the surface, so pressure only cleans above it and the stain migrates back within days. Removing oil requires chemistry to emulsify it and a poultice to draw the deep portion out.
What is the best oil stain remover for concrete?
An alkaline concrete degreaser handles most stains up to a few weeks old. For older stains, a poultice made from an absorbent powder and a solvent is the effective method. Microbial oil removers also work well on old stains if you can leave them damp for days.
Does cat litter remove oil from concrete?
Clay-based cat litter absorbs a fresh spill effectively if applied before the oil soaks in and left for several hours. On a stain that has already penetrated, it does little on its own, but ground fine and mixed with a solvent it makes a serviceable poultice.
Can I use bleach on an oil stain?
Bleach does not remove oil, and it introduces a real hazard, because it must never be combined with the acidic or solvent-based products used on concrete stains. Bleach and acid together release chlorine gas. Use a degreaser or a poultice instead.
The whole-house equivalent of this routine is covered by house cleaning services.
Old oil is a patience problem more than a technique problem, and the poultice cycles do the work that pressure cannot. If the staining covers enough of the slab that evenness is the real issue, get a quote and we will look at the whole surface.