Oil on concrete is not a stain so much as a resident. It sits in the pore network, it darkens when it rains, and it laughs at the first pass of a cheap degreaser poured from a supermarket bottle at noon. Mould is a different resident: a film that likes the south wall, the side path, the lake court under the pittosporum, any slab that stays wet enough to be a climate. Both are cleaning problems. Both become durability problems if you attack them with a machine that removes the paste, or with a chemistry that leaves the slab more open than you found it.
This is the domestic end of repairs and resurfacing, and it belongs next to coastal durability because in Newcastle the gutter is not a theoretical object. It is a pipe with a harbour at the end of it, or a lake, or the Hunter. What you put on the driveway is a vote about that water.
Oil: what is still on top, what has gone in
Fresh oil — a Sunday drip, a tipped can — should be absorbed, not washed. Use sand, cat litter, a commercial absorbent, anything that will hold the liquid before it becomes a ghost in the slab. Sweep it. Dispose of it as waste, not as garden mineral. The longer a petroleum sits, the further it travels into the pores, and the more it will reappear as a shadow every time the concrete wets.
What remains after the free oil is gone is a candidate for a degreaser or a poultice. A degreaser, alkaline and meant for masonry, works on the oil that is still near the surface. You apply it, you give it time, you agitate with a stiff brush, you lift the emulsion with rags or a wet-vac, and only then do you rinse. The point of the sequence is to pick the oil up, not to chase it into the expansion joint and congratulate yourself because the surface looks briefly matte.
A poultice is slower and, on an old gearbox spot, often more honest. Pack an absorbent with an alkaline cleaner on the stain, cover it, and let the drying pull the oil out. Repeat. Deep, decade-old stains will lighten and will not vanish. Anyone who promises vanishing on a 1998 Holden drip is selling you a different driveway. Solvent thins the oil, which can look like success, and then carries it further in or off the slab into the pit. Warehouse floors are the same chemistry at a larger scale, often sealed or painted. Do not assume a method that worked on a factory in Tomago will be kind to a stamped court in Merewether.
Pressure washing, which is a tool and a vice
A pressure washer is a good way to lift film, algae and the last of a detergent if the tip is a fan, the distance is respectful, and the concrete is sound. It is a bad way to “get the oil out”. Pressure will drive some oil deeper, open the surface paste, and leave a halo that is cleaner and more absorbent than the rest of the slab. The next drip will look worse. The next mould will arrive sooner.
Turbo nozzles on a 1980s decorative finish expose aggregate you did not mean to expose. If the slab is young, you should be curing it, not washing it. If it is old and friable, water at pressure is demolition. Surface preparation before an overlay or a reseal is grinding by other means; the alkaline slurry belongs in a settling drum, not the gutter. If you hire a trailer washer, brief the operator: fan tip, no turbo on decorative work, control the first dirty water.
Mould, shade, the south side
In New South Wales the south face of a building is the one the winter sun forgets. Add a fence, a dense shrub, a lake humidity, a leaking gutter, and you have a climate. The black film on the concrete is usually a mix of mould, algae, dirt and the organic soup that a damp mineral collects. It is slippery. It is ugly. It is not, by itself, a failed slab.
Cleaning it without changing the wet is a subscription. Cut the shrub, fix the gutter, then clean. A masonry-safe mould wash with a controlled rinse will do more than bleach poured until the air hurts. If you use a chlorinated cleaner, keep it on the slab — not in the pit, not in the street when the next east-coast low is due. On the lake, the lakeside essay is the climate chapter. On the ocean streets, do not acid-wash a balcony soffit you then have to inspect for rust. A film-forming sealer on a slab in permanent shade can trap moisture and feed the next crop.
Runoff is the harbour
Newcastle’s stormwater is not a treatment plant. It is a network that takes the street to the basin, the harbour, the beaches, the lake. Oil emulsion, paint flakes, acid rinse, chlorinated foam, the slurry from a grind: these are the things a careful job keeps on the block. The legal detail sits with the councils and with the state. The ethic sits with you. If you would not pour it on Merewether Beach, do not pour it on a driveway that drains to the same water.
Plug the pit. Work in small bays. Wet-vac the first lift of dirty water. If a contractor cannot tell you where the runoff will go, they have not priced the job. Wash a boat on a slab that falls to a point you control, not toward the foreshore. The lake is a receiving water. So is the harbour.
What not to use
Hydrochloric acid, used as a default cleaner, is a tradition that has cost a lot of paste. It will remove some stains. It will also etch, open pores, leave a yellow or rust mark if you are careless with the metal, and make the next stain easier. There are jobs for acid — a skilled etch before a coating, a specific rust treatment in experienced hands. A habit of “just a bit of spirit of salts” on every driveway is not one of them.
Muriatic bravado, paint stripper that was meant for timber, oven cleaner, a mystery solvent from the back of the shed, a “concrete brightener” with no safety data, a sealer slapped on a wet oily slab to “lock the stain in”: these are the short list. So is any product whose first instruction you cannot follow on a slope that drains to the street.
Abrasive pads on a decorative finish, wire wheels on a balcony edge, sand on a rubber broom used as if the slab were a ship's deck: these remove the cream and leave a surface that will stain faster. If you are preparing for an overlay, abrasion is a specified step. If you are cleaning, it is usually a mistake.
A working method, then a stop
Absorb, then chemistry, then gentle water, then dry, then decide whether the remaining shadow is something you can live with. For mould, change the shade and the wet, then clean, then wait to see if you bought a season or a week. For both, keep the mess on the slab until you have a legal, boring place for it to go.
If the edges are hollow or the steel is talking, stop cleaning and start reading the rest of this hub. If the slab is sound and the stain is the whole complaint, you are allowed to stop before perfection. Concrete in this climate is a working mineral. The job is to keep the paste, keep the harbour out of the story, and refuse the machine that makes a cleaner-looking ruin. Poultices work overnight. Mould returns if the gutter still leaks. In Newcastle the end of the driveway is not the end of the water.
