There’s a stretch of Purley Way where the dealerships run one into the next, and you can read every one of their cleaning contracts off the concrete without getting out of the van. The forecourt with the even, slightly chalky grey has been acid-washed at some point and never recovered. The one with the dark halos under each delivery bay has been jet-washed hard and often, by somebody who thought pressure was the answer. And there’s one – I won’t say which marque – where the slab still looks like a slab, twelve years old, sixty cars a week rolling across it, and the only thing on it is tyre marks.
That third one isn’t luck. Oil on concrete is a chemistry problem that people insist on treating as a pressure problem, and the difference between the sites that stay clean and the sites that go grey and pitted is almost entirely about what gets applied, how long it’s left, and whether anyone bothered to seal the thing.
Why does oil sink into concrete instead of sitting on it?
Concrete looks solid. It’s a sponge with delusions. Even a well-finished power-floated slab has a capillary network running down through it, and the pores at the surface are somewhere between a few nanometres and a few microns across – small, but oil is thin and patient and gravity has all week.
Fresh oil wicks in fast. A hydraulic leak from a low-loader that sat for forty minutes on an unsealed bay is already 2-3mm into the slab by the time somebody finds it. That’s the part people get wrong when they arrive with a jet wash: the stain they can see is the shadow of oil that’s now below the surface, and blasting the surface removes the top layer of cement paste along with a bit of the discolouration, which leaves the slab lighter and thirstier for the next spill.
Then it changes. Motor oil left in concrete oxidises and polymerises over weeks – it thickens and darkens, and it stops behaving like a liquid and starts behaving like a resin sat inside the pore structure. Nine times out of ten, when a site manager says “we’ve tried everything on that one”, the stain is eighteen months old and nothing short of a poultice was ever going to shift it.
The forty-eight hour window nobody uses
Everything in this trade is easier in the first two days. Granular absorbent down immediately, swept up rather than ground in with a boot, then a degreaser and warm water while the oil is still mobile – that’s a ten-minute job that stays gone. The same spill at six weeks is a two-hour job with a 60% result. Yet almost no forecourt has absorbent within thirty seconds’ walk of the bays where spills happen, which tells you how seriously the forty-eight hour window gets taken.
What actually lifts the oil – the chemistry or the pressure?
Chemistry. It isn’t close.
Oil doesn’t dissolve in water, so the whole job is getting the two to talk to each other. That’s what a surfactant does: one end of the molecule likes oil, the other likes water, and it lifts the oil into suspension so it can be rinsed. Alkaline degreasers – sodium hydroxide or potassium hydroxide based, pH 12 to 13 – go further and saponify the oil, turning the fatty fraction into a crude soap that rinses off with the water.
Dwell time is the whole game. A strong alkaline degreaser at four minutes does less than a mild one at fifteen. The single most common mistake I see contract cleaners make on a forecourt is applying product and rinsing it within ninety seconds because the machine is already running and standing about looks like idling.
Bio degreasers – the enzyme and microbe products – are where I’ll lose some readers. They do work. They eat the hydrocarbon and leave water and CO2, and on a slab you can leave damp and undisturbed for three weeks, they’re the best thing available. No petrol station in Greater London can leave a bay damp and undisturbed for three weeks. Most of these products are sold on conscience rather than performance, and I’ve watched two sites pay a premium for something that never had a chance of working within the trading hours they keep.
Hot water is the multiplier
Heat halves oil viscosity roughly every twenty degrees. A 90°C machine with a modest 80 bar at the tip will out-clean a cold 200-bar unit on any oil job, and it will do it without touching the slab. That’s the trade-off worth buying: temperature over pressure. Steam-capable units go further still on cured deposits, though you’ll want somebody who knows what a steam lance does to a shin.
How do you clean a forecourt without ending up in front of the Environment Agency?
The wash water is the legally interesting part. A forecourt’s drainage runs to a separator, and the moment you introduce a degreaser you’ve made it harder for that separator to do its job – emulsified oil doesn’t rise out of water the way free oil does, which is what a coalescing separator relies on.
Petrol filling stations have interceptors as standard. Car dealerships and retail forecourts frequently do not, and their surface water gutters run straight to a watercourse. Contractors who wash oil into those drains are committing a water pollution offence, and the fines are not small.
The compliant version is dull and it costs money: bunded matting or a drain seal round the gully, wash water recovered with a wet vac or a recovery ring, then tankered or discharged to foul sewer under a trade effluent consent from Thames Water. On a small dealership forecourt off Western Avenue that’s another two hours and a disposal cost, which is exactly why the quote that comes in at half the price is the quote that isn’t doing it.
Ask what the separator is rated for
Class 1 separators go down to 5mg/litre of residual oil; Class 2 to 100mg/litre. Sites with a Class 2 unit and a hard-standing where cars are washed are already marginal before a contractor turns up with a drum of degreaser. Reading the site drainage plan before quoting takes fifteen minutes and has saved me from two jobs I’m glad I didn’t take.
What actually damages the concrete on these jobs?
Acid. Every time. Somebody, somewhere, decided that hydrochloric acid at 10% is a concrete cleaner, and it’s still being poured on London forecourts by people who should know better. It leaves the oil exactly where it was and dissolves the calcium hydroxide and cement paste holding the aggregate together, so the stain goes because the surface goes. What’s underneath is a raw, open, sugary slab with roughly double the porosity it had that morning, and it will stain darker and faster forever after. Acid-washing a forecourt should end a contract on the spot, and I’ve said so to a facilities manager’s face in Park Royal.
The pressure damage is subtler. A turbo nozzle held close will cut visible rings into a power-floated surface, and a rotary flat-surface cleaner run too slow over green concrete will strip the fines and leave you looking at exposed sand. Under 150 bar on the slab, and hotter water through a wider tip – the concrete doesn’t care about pressure nearly as much as it cares about acid.
Poultices for the stains that won’t go
For old, polymerised stains, a poultice is the only honest answer. A solvent or degreaser carried in an inert absorbent – diatomaceous earth, or flour on a small one – troweled on 5-8mm thick, covered with polythene taped at the edges, left for twenty-four to forty-eight hours. It draws oil back out as it dries. It’ll take two applications and it might get you 80%. Nine times out of ten that’s better than anything a machine will do, and it costs nothing but patience.
Why do the same bays stain again a month later?
Because unsealed concrete is a stain waiting for a customer. Cleaning a slab and leaving it bare is doing free absorbency testing on your own forecourt.
Silane-siloxane penetrating sealers are the standard answer – they go into the pores and line them without forming a film, so the slab keeps its skid resistance and its appearance and stops drinking. Two coats, reapplied every three to five years on a trafficked forecourt. A lithium silicate densifier alongside will tighten the paste and make the surface harder to fret.
The three-year cycle beats the five-year quote
Sealer manufacturers quote longevity from lab abrasion figures, and a forecourt is not a lab. Power steering scrub in a turning bay chews a silane-siloxane far faster than the datasheet allows for, so the bays by the entrance are bare again long before the middle of the slab is. The fix is boring: seal the whole thing, then re-do the two or three worst bays on a shorter cycle, which costs a morning and a few litres. A site that does this never has a dramatic stain to photograph, which is a problem, because facilities managers approve budgets for problems they can photograph. I’ve lost renewals to sites that let their slabs rot and then paid three times my figure to a company promising a miracle.
The dealership on Purley Way with the twelve-year-old slab has its wash-down bays sealed on a rolling three-year cycle and absorbent in a bin at each corner. That’s the whole secret, and it has nothing to do with machines or chemicals. They treat the concrete as something they own rather than something they walk on.
Nine times out of ten, the site that never needs a heroic clean is the site that paid for sealer eleven years ago and has never mentioned it since.
