Oil & Grease Stains

Oil is a depth problem, not a surface problem

Concrete looks solid. It is not. A standard residential slab is a rigid sponge threaded with capillary pores, and anything liquid that lands on it starts travelling downward the same day. Motor oil is thin, persistent, and has months to work with.

So the dark spot you are looking at is not the stain. It is the top of a column. Under that circle of discoloration there is contaminated concrete reaching a quarter inch down or more, and that buried oil keeps migrating back toward the surface every time the slab warms up. This is the whole reason people think oil stains are unremovable: they clean the top, it looks good for a weekend, then the stain reappears out of nowhere. Nothing reappeared. It was never gone.

Which means the correct method is the opposite of what instinct suggests. Blasting harder does not reach deeper. At 2,500 to 4,000 PSI a pressure washer erodes the cement paste off the top of the aggregate and leaves a clean-looking crater — and the exposed surface inside that crater is rougher and more absorbent than the slab was before, so the next drip soaks in faster and spreads wider. You can make a driveway permanently worse at oil resistance in about ninety seconds.

Oil comes out by dwell and lift. You put chemistry on it that will emulsify the hydrocarbons, you give it time, you add heat, and you draw the oil upward out of the pores. Then you do it again.

The stain tells you what spilled

Not all dark spots are the same contaminant, and the treatment changes with each one.

Fresh motor oil. A defined dark circle with a soft halo, often still slightly tacky. The best outcome available in this whole category, because the oil has not oxidized or travelled far. Blot it, cover it with absorbent clay, and get it treated inside the week.

Old oxidized oil. Months or years of small drips in the same parking spot. The slab goes evenly dark rather than spotty, often with a bluish or grey-brown cast, and the edges blur. The oil has polymerized and no longer dissolves as readily, so this is the one that needs repeat cycles and a poultice.

Transmission and hydraulic fluid. Thinner than motor oil, dyed red or amber, and it penetrates faster and further for exactly that reason. The visible stain is often smaller than the contaminated area underneath it.

Cooking oil and kitchen grease. The commercial case — restaurant dumpster pads, back-door flatwork, grease interceptor surrounds. Vegetable and animal fats go rancid and feed bacteria, so these pads carry an odour problem and a slip-hazard problem alongside the stain. They respond well to an alkaline degreaser and hot water, but they need a schedule rather than a single visit.

Asphalt sealer tracking. Black footprints or tyre tracks carried from a newly sealcoated street onto a concrete driveway. People assume it is oil. It is a tar-based coating sitting on the surface, and it comes off with a citrus or d-limonene solvent rather than an alkaline degreaser — the right diagnosis saves pointless scrubbing.

How we treat it

A surfactant-based industrial degreaser goes on the stain at working concentration, applied to a dry surface so it penetrates rather than floats, with a dwell of 15 to 30 minutes. The surfactant breaks the oil into droplets that water can carry. Then hot water — heat drops the viscosity of the oil and does more for lift than pressure ever will — with mechanical agitation only as needed, and a controlled-pressure rinse through a surface cleaner so the lifted oil is captured and carried off rather than spread into a wider grey shadow.

Then we read the result and go again. Most meaningful oil stains take two or three passes in one visit, each one pulling the next layer of the column up.

For deep, old staining we switch to a poultice: degreaser carried in an absorbent clay or diatomaceous earth paste, troweled over the stain and left to dry for several hours. As the water evaporates out of the poultice it draws the dissolved oil up with it and traps it in the clay, which then gets scraped off and disposed of. It is slow, it looks unglamorous, and it is the only thing that reliably reaches a decade-old stain.

Runoff gets managed throughout. Hydrocarbons do not belong in a storm drain, and on Sonoma Valley driveways that drain to the street that is an active consideration rather than a footnote.

Sealing is the actual prevention

Once the concrete is as clean as it is going to get, the question becomes what happens to the next drip. On unsealed concrete, the answer is the same thing that happened to the last one.

A penetrating silane-siloxane sealer changes that. It lines the pore walls, keeps oil at the surface where a hose and a squirt of degreaser can handle it in two minutes, and does not form a film that peels. It needs genuinely dry concrete to bond — budget 48 to 72 hours of drying in Sonoma summer weather and longer for a shaded driveway under oak canopy — so it is a return visit, not the same afternoon. Plan on reapplication every three to five years on a driveway that sees daily traffic.

What this will not fix

The honest limits, said before the work rather than after:

• Spalling, scaling, or pitting. If the top layer of the slab is already delaminating, cleaning exposes more of it. That is concrete repair.
• Etched craters from a previous pressure job. Permanent. We can clean inside them; we cannot restore the surface profile.
• A decade-old parking shadow going to invisible. Expect dramatic improvement and a lighter ghost. Anyone promising otherwise has not seen your driveway.
• Oil under a failing sealer. The old coating has to come off first, which changes the scope and the price.
• Rust and tannin staining. Often on the same slab, completely different chemistry — see rust stains.

Pricing

• From $325 for residential driveway stains
• Typical project $325-$495

Commercial flatwork — restaurant pads, fleet parking, shop floors — sits at the upper end of that range for a single treatment, and is usually better handled on a recurring schedule than as one heroic visit. Stain work is normally quoted alongside a full driveway and concrete clean or a wider pressure wash, because a treated spot inside an untouched slab reads as a new stain of its own.

Questions we get

Can an old oil stain be completely removed from concrete?

A fresh spill treated within a few days is usually a full removal, because the oil is still in the top millimetre or two and a degreaser with hot water can reach all of it. A ten-year-old parking-spot shadow on unsealed concrete is a substantial-improvement job — realistically a 60-85% reduction over two or three treatment cycles — because the oil has wicked a quarter inch or more into the slab. Anyone promising perfection on a decade-old stain without seeing it is guessing.

Does kitty litter or baking soda get oil out of a driveway?

Absorbent clay works well on a spill that is still wet and has not soaked in yet, which is why it belongs in the garage before it belongs on a quote. Once the oil has penetrated, scattering litter on top does nothing, because there is no liquid left at the surface to absorb. At that point you need a surfactant degreaser with a 15-30 minute dwell and hot water, or a poultice drawn across several hours, to pull the oil back up out of the pores.

How long can an oil stain sit before it becomes permanent?

It is not a hard cutoff, it is a gradient — every week the oil migrates deeper and oxidizes further, and oxidized oil is harder to re-dissolve than fresh oil. Practically, a spill addressed in the first week usually disappears, one addressed in the first season usually comes out well, and anything past a year or two is a reduction job rather than a removal job. If you have a fresh spill right now, blot it, cover it with absorbent clay, and book sooner rather than waiting for a convenient date.

Will pressure washing get oil out of concrete?

Not by itself. Oil is a depth problem and pressure is a surface tool, so 3,000 PSI knocks the dirt off the top of the stain and makes it look briefly better while leaving the oil column underneath — which is why the stain reappears within days as it wicks back up. Worse, high pressure erodes the cement paste and leaves an etched crater that is more absorbent than the slab around it, so the next drip stains faster than the last one. Sodium hypochlorite is no help either; chlorine kills biological growth and does nothing to hydrocarbons.

Should I seal my driveway after the oil stains are removed?

Yes, if you want the result to hold. Unsealed concrete is an open sponge and will simply absorb the next drip, whereas a penetrating silane-siloxane sealer keeps oil at the surface where a hose and a degreaser can handle it. Sealing has to happen on genuinely dry concrete, so plan on 48 to 72 hours of drying after the wash in Sonoma summer weather and longer in shade — which means it is a separate visit, not the same afternoon.