TL;DR
Absorbent first, degreaser second -- never the other way around. Loose oil has to be pulled up with a granular absorbent before any liquid touches it, or the degreaser simply spreads the stain wider and drives it deeper. Concrete is porous, so an old stain that has soaked in for years often lightens substantially but never fully leaves, and that is a realistic expectation rather than a failure.
Why concrete makes this hard
Concrete looks solid and behaves like a sponge.
It is full of interconnected capillary pores, and oil wicks into them the way water wicks into a paper towel. What you see on the surface is the top of a stain that may extend a quarter inch or more into the slab.
That geometry explains everything about the method. A surface cleaner cannot reach oil below the surface. Scrubbing only cleans the top layer, and the oil beneath then migrates back up as the slab dries, which is why a stain often looks great when wet and reappears the next day.
The other consequence is that fresh spills are a completely different problem from old ones. Oil spilled ten minutes ago is mostly still on top. Oil spilled two years ago is in the slab.
Source: Concrete cleaning and restoration practice
What is the right order for cleaning oil off a garage floor?
Cover the spill with a granular absorbent and let it draw the oil up, sweep it away, then apply an alkaline degreaser and let it dwell, agitate with a stiff brush, and rinse. For old stains that soaked in years ago, follow with a poultice that pulls oil back out of the pores over 24 to 48 hours.
Step one: absorbent, before anything wet
This is the step people skip and it is the one that decides the outcome.
Cover the area generously with a granular absorbent -- clay-based absorbent, diatomaceous earth, unscented cat litter, sawdust, or even cornstarch on a small spill. Grind it in underfoot or with a stiff brush so it makes contact.
Leave it. Thirty minutes on a fresh spill, overnight on a heavy one. The granules wick oil out by capillary action, which is the only mechanism that moves oil upward out of concrete without chemistry.
Sweep it up and dispose of it appropriately -- oil-soaked absorbent is not ordinary trash in many places, and your local waste authority sets the rule.
Why order matters. Pour a degreaser onto free-standing oil and you make an emulsion that flows into a wider area and further down. You have turned a small dark spot into a large grey one.
Step two: degreaser and dwell
Now the chemistry has a chance.
Use an alkaline degreaser -- a purpose-made concrete degreaser, a heavy-duty alkaline cleaner, or a strong solution of a general degreasing product. Alkaline works because it saponifies the oil, turning it into something water can carry away.
Apply it to a slightly damp floor so it does not immediately soak in. Keep it wet for the label dwell time, which is commonly 10 to 20 minutes. Mist it if it starts to dry. Dwell is where the work happens; adding more product is not a substitute for adding more time.
Agitate with a stiff-bristled deck brush or a floor scrubber. Work in overlapping passes across the whole stain and a margin around it, since a cleaned circle inside a dirty floor is very visible.
Rinse thoroughly and squeegee the water out toward the door.
Step three: a poultice for what is left
For a stain that has been there for years, a poultice is what reaches into the pores.
Mix an absorbent powder with a solvent or degreaser into a paste roughly the consistency of peanut butter. Spread it a quarter inch thick over the stain, extending past its edges. Cover with plastic and tape the edges down.
Leave it 24 to 48 hours, then let it dry uncovered before scraping it up. As the poultice dries, it pulls the dissolved oil out of the concrete and into itself. That is the reverse of the direction oil normally moves, and it is the only method that genuinely gets below the surface.
Repeat cycles produce progressive lightening. There is more detail on stubborn stains in removing oil stains from concrete.
Chemical safety on a garage floor
Never mix a chlorine bleach product with an acidic cleaner, a rust remover, or an ammonia-based degreaser. Bleach plus acid releases chlorine gas, and bleach plus ammonia releases chloramine gas.
A garage is the worst possible place to make this mistake, because the door tends to be down, the space is enclosed, and you are working on your hands and knees at floor level where heavier gases collect. Open the door fully, work one product at a time, and rinse completely between products.
Also keep solvent products away from any ignition source, including a water heater or furnace pilot light in the same space. Solvent vapors are heavier than air and travel along the floor.
What to expect, honestly
A fresh spill treated within an hour usually disappears completely. A stain a few months old lightens dramatically. A stain that has been soaking in for years typically lightens by a lot and leaves a shadow, and that shadow may be permanent.
Which is why prevention is worth more than any product. Put a drip pan under a vehicle with a known leak, use a garage floor mat under the parking spot, and seal bare concrete with a penetrating sealer so the next spill sits on top rather than soaking in. A sealed floor is a vastly easier floor. Epoxy and coated floors have their own rules, covered in caring for an epoxy garage floor, and winter brings a different residue problem in removing salt and de-icer residue.
Routine care once it is clean
The general garage floor routine is covered in cleaning a garage floor, and how the garage fits with the rest of the exterior in the exterior surface cleaning overview.
FAQ
What is the best degreaser for a garage floor?
An alkaline degreaser made for concrete, because alkaline chemistry breaks oil into something water can carry away. Apply it to a slightly damp floor, keep it wet for the full label dwell time, and agitate with a stiff brush. Dwell time matters more than product strength.
Should I use cat litter or a degreaser first?
Absorbent first, always. Granular absorbent pulls free oil up and out of the concrete, and that is the only way to remove what is sitting loose. Pouring degreaser onto standing oil creates an emulsion that spreads the stain wider and pushes it deeper into the slab.
Can old oil stains be removed from concrete completely?
Often not. Oil wicks into concrete's capillary pores, and a stain that has soaked in for years typically lightens substantially but leaves a shadow that may be permanent. Repeated poultice treatments produce the best results. Resurfacing or coating is the option when the shadow is unacceptable.
What is a poultice and how does it work?
A paste of absorbent powder mixed with a solvent or degreaser, spread a quarter inch thick over the stain and covered with taped-down plastic for 24 to 48 hours. As it dries it draws dissolved oil out of the concrete pores and into itself, reversing the direction the oil traveled.
Can I pressure wash a garage floor?
Yes, and hot water with a surface cleaner attachment works well on surface grime. Two cautions: the wash water has to be collected or drained legally, since oily runoff into a storm drain is prohibited in many places, and pressure does not reach oil that has soaked below the surface.
Is it safe to mix cleaners to get a stain out?
No. Never combine a bleach product with an acidic cleaner, a rust remover, or an ammonia-based degreaser. A garage is an especially bad place for this, since the door is often down and you are working at floor level. Use one product, rinse fully, then try another.
How do I stop the garage floor from staining again?
Put a drip pan under any vehicle with a known leak, use a floor mat under the parking spot, wipe fresh drips immediately, and seal bare concrete with a penetrating sealer. A sealed floor keeps the next spill on the surface, where it wipes up instead of soaking in.
The whole-house equivalent of this routine is covered by house cleaning services.
Floor size, how long the stains have been there, whether the slab is sealed, and how the rinse water can be handled all change what the job takes, and some stains are a resurfacing conversation. Get a quote and we will look at yours and be straight about what is achievable.