Short answer. To remove an oil stain from concrete, absorb any wet oil first, apply a degreaser and let it dwell, then wash with hot water. Fresh stains usually come out completely. Stains that have soaked in for years will lighten but may need sandblasting to remove.
Why oil is so hard to get out of concrete
Concrete is porous. Oil does not sit on top of it for long. It wicks down into the surface, and heat speeds that up. A drip that lands on a parking stall in an Inland Empire summer is into the slab within days.
That is why age matters more than size. A large fresh spill is easier to remove than a small stain that has baked for three years.
Step 1: absorb what is still wet
If the oil is still liquid, do not hose it. Water spreads oil and carries it to the nearest drain. Cover it with absorbent, cat litter or even sand, leave it, then sweep it up and bag it.
Step 2: degrease and wait
A concrete degreaser breaks the bond between oil and the surface. The mistake most people make is rinsing too soon. Give it time to work, and scrub it in on heavy spots so it reaches into the pores.
Step 3: hot water, not just pressure
Cold water at high pressure moves loose dirt and leaves oil behind. Hot water softens the oil so it releases. On commercial sites we wash with hot water through a flat surface cleaner, which also keeps the finish even instead of leaving wand stripes.
On a job in Orange County, a parking spot stained with oil, diesel and dirt went back to looking new after one late-night hot wash.
Step 4: when washing is not enough
Some stains have bonded with the concrete. Hot water will lighten them and then stop. At that point the options are to live with a shadow or remove the top layer of the surface.
Sandblasting does the second. We used it on a Chevron where rust and tire rubber had locked into the fuel pads after another crew failed to wash them clean. The slabs came back to a uniform color.
What does not work
A few things we are asked about often.
- Bleach. It does nothing to oil.
- Cold pressure washing alone. It cleans around the stain and makes it stand out more.
- Hosing a fresh spill. It spreads the oil and sends it to the storm drain, which in California can mean a citation.
- Acid. It etches the concrete and leaves the oil.
For commercial properties: recover the water
Wash water that carries oil is not allowed to enter a storm drain in California. On a commercial site the drains should be blocked and the water collected. That is standard on every oil job we do.
