A garage floor accumulates a specific set of problems that a driveway does not, mostly because it never gets rained on and never fully dries.
Three stains account for nearly all of it, and they need three different approaches.
Oil
Oil in a garage slab is deep. Years of slow drips, never washed away, soaking into unsealed concrete.
Pressure does not remove it — the oil is in the pores, not on the surface. What removes it is chemistry plus dwell time.
Fresh spills: cover with absorbent material — cat litter, sawdust, commercial absorbent — and leave it to draw the oil up before doing anything wet. Sweep up, then degrease.
Established stains: an alkaline degreaser applied neat, agitated with a stiff brush, left for the stated dwell time, then rinsed. Expect to repeat. One application rarely does it.
Deep old stains: a poultice — an absorbent paste mixed with solvent, spread over the stain, covered with plastic, and left 24 hours to draw the oil back out as it dries. Slow, and the only thing that works on genuinely old contamination.
The same method applies outdoors, covered in how to remove oil stains from a driveway.
Enzyme and bacterial degreasers are worth knowing about. They work slowly — days rather than minutes — by consuming the hydrocarbons, and they handle deep contamination better than surfactants do.
Tyre marks
Not dirt. These are plasticisers migrating out of the tyre rubber into the concrete, accelerated by heat, which is why a car parked in the same spot all summer leaves a permanent-looking patch.
Degreaser touches them only partially. A citrus or solvent-based cleaner works better, with scrubbing. Expect improvement rather than complete removal on an unsealed slab — the plasticiser has gone into the concrete.
Sealing is the actual answer, because it prevents migration in the first place.
Worth knowing: this is also why hot tyres mark a freshly sealed acrylic floor. Parking on a coating before it has fully hardened transfers the print permanently — covered in choosing a concrete sealer.
Efflorescence
The white powdery bloom, usually along the back wall or in patches across the floor.
This one is a message rather than a stain. It means water is moving up through the slab, dissolving salts, and evaporating at the surface. The salt is left behind.
Brush it off dry. Wetting it redissolves the salts and carries them back into the concrete, where they reappear within weeks. This is the most common mistake and it is worth getting right — the mechanism is the same as on brickwork.
Recurring efflorescence on a garage floor usually means no vapour barrier under the slab, which is common in older Sydney garages. It also means a coating applied over the top will fail, which is why the moisture test matters before any epoxy goes down — see prepping a garage floor for epoxy.
Dusting
If the floor constantly produces a fine grey powder and everything in the garage gets a film on it, the slab has a weak laitance layer at the surface — usually a finishing or curing problem from when it was poured.
Washing does not fix it; it exposes more. The fix is a silicate densifier, which reacts with free lime in the concrete to harden the surface, or grinding the weak layer off.
Doing it without flooding the garage
The practical problem indoors is that there is nowhere for the water to go, and you cannot just hose it out the door.
Options: a wet vacuum to recover the slurry, which also keeps the runoff out of the stormwater system; squeegeeing to a floor waste if there is one; or working in sections.
Runoff carrying degreaser and oil must not go to the stormwater drain, and a garage usually drains straight to the driveway and then the kerb. That is a pollution offence in NSW and the rules are in where the dirty water goes.
Ventilate as well. Solvent-based cleaners in a closed garage build up quickly.
Then seal it
Once the floor is actually clean, sealing is what stops the cycle restarting.
For a garage the usual options are a silicate densifier to harden and reduce dusting, a penetrating sealer for stain resistance with no film to peel, or an epoxy or polyurethane coating for the full finish.
Run the plastic sheet moisture test first regardless of which you choose. A slab with active moisture migration will reject a coating.
For the cleaning and degreasing stage, see concrete cleaning.
