Dark patches on concrete straight after pressure washing are usually just uneven drying, and they disappear within a few hours. If they're still there once the whole slab is bone dry, you're looking at one of four real causes: wand etching, exposed aggregate, a stain the cleaning has revealed, or residual moisture from below. Each has a different fix, and one of them can't be fixed at all.
Below: why you should wait before panicking, the five causes, how to tell etching from a stain, what fixes each, and how to avoid it entirely.
First: wait for it to dry
This resolves the majority of the calls we get about this, and it costs nothing.
Concrete is porous, and it doesn't hold or release water evenly. Thicker sections, shaded sections, areas over a different sub-base, and areas where the finisher worked the surface harder all dry at different rates. For an hour or two after washing, a slab can look genuinely blotchy — and then look perfectly uniform once it's finished drying.
- Surface dry: two to four hours in typical Jacksonville weather.
- Full moisture release: up to 24 hours, longer on a thick slab or a humid day.
- Shaded sections can lag the sunny ones by several hours — which is itself a common cause of apparent "patches".
So: look at it the next morning, in similar light, before deciding anything is wrong.
The five real causes
| Cause | How to recognise it | Fixable? |
|---|---|---|
| Uneven drying | Fades as the slab dries; gone within hours | Resolves itself |
| Wand etching | Sharp-edged lines or arcs matching the wand's path | No — only blend or resurface |
| Exposed aggregate | Rough to touch; you can see the stones | No — resurface only |
| Stain revealed by cleaning | Soft, irregular edges; often oil or organic | Yes — chemical treatment |
| Rising damp / poor drainage | Reappears after rain, fades in dry weather | Yes — fix the drainage |
Wand etching and burn marks
This is the one nobody wants, and it's the most common genuine damage from pressure washing concrete.
Concrete has a thin layer of cement paste at the surface — the smooth part. Beneath that is the aggregate. Hold a narrow-angle nozzle too close, or pause with the wand in one place, and you erode that paste layer and expose the rougher, darker material underneath.
The giveaway is the shape. Etching follows the operator: straight lines, arcs, or a distinct teardrop where someone stopped moving. Natural staining never has sharp, geometric edges. Run your hand over it — etched concrete feels noticeably rougher than the surrounding slab.
It can't be reversed. The options are:
- Blend it. Clean the entire slab evenly with a rotary surface cleaner so the whole area matches. Works for light etching and is what we'd try first.
- Resurface. A cement-based overlay across the full slab. Effective, but it's a resurfacing job, not a cleaning one.
- Stain or coat. A solid-colour concrete stain or coating hides texture variation. Commits you to maintaining a coating.
New concrete is the most vulnerable — under about twelve months it hasn't reached full strength. It's also why we use a surface cleaner rather than a wand on open slabs: the housing keeps a fixed standoff, so you physically can't linger the way you can with a wand.
Stains revealed by cleaning
This one catches people out because it feels like the washing caused it. It didn't — it just raised the contrast.
Oil, rust and organic staining sit deeper than surface grime. When the grime is removed the surrounding concrete gets several shades lighter, and a stain you'd stopped noticing suddenly stands out. The stain was always there; the background changed.
Each needs its own chemistry, not more pressure:
- Oil and transmission fluid — degreaser, agitation, dwell, then rinse. Older stains need a poultice to draw oil back out of the pores. Expect 70–95% removal, not perfection.
- Rust from irrigation — oxalic-acid based remover. Extremely common on well-water systems in St. Johns and Nocatee.
- Tannin from oak leaves — brown, not green. Needs an alkaline pre-treat, not bleach. Endemic in Mandarin.
- Battery acid — the discolouration can be neutralised but the pitting is permanent.
What fixes each one
| Cause | Fix | Typical cost |
|---|---|---|
| Uneven drying | Wait 24 hours | $0 |
| Light etching | Full-slab even re-clean with a surface cleaner | $129 – $209 |
| Deep etching / exposed aggregate | Concrete resurfacer or overlay | $3 – $8 / sq ft |
| Oil stain | Degreaser + poultice, possibly twice | From $45 |
| Rust stain | Oxalic-acid rust remover | From $45 |
| Rising damp | Drainage correction, then re-clean | Varies |
Preventing it
- Use a rotary surface cleaner on any open slab. Constant standoff, constant speed, no lingering.
- Keep the wand for edges and detail only, at a 25–40° tip and at least 12 inches back.
- Never stop moving. If you have to pause, release the trigger first.
- Pre-treat organic growth chemically so you don't need to compensate with pressure.
- Go gentler on concrete under 12 months old.
- Clean the whole slab, not just the dirty part. Partial cleaning is how you create a visible line you then have to chase.
My honest take
Nine times out of ten this is a false alarm and the customer is looking at a slab that hasn't finished drying. If you've just had your driveway done and it looks patchy, make a coffee and look again tomorrow morning.
The tenth time, it's usually a hand wand in the hands of someone being paid by the job. Wand marks on concrete are the single clearest sign of a rushed operator, and they're the reason we won't clean an open slab with anything but a surface cleaner — it's not that our technicians are better, it's that the tool removes the possibility of the mistake.
If it is etched, be realistic. Anyone promising to "clean out" a wand mark is going to make it worse by hitting it harder. The honest answer is blend the whole slab and accept a slightly different texture, or resurface. We'd rather tell you that than take your money twice.
Related reading: how to pressure wash a driveway properly and what to ask before hiring anyone. If you've got patches you can't identify, send us a photo and we'll tell you which of the five it is.
