The short answer
A stiff brush and a decking cleaner, brushing along the grain. A pressure washer raises the grain and turns smooth decking into something splintery that traps dirt.
- Sweep, and clear the gaps between boards with a thin blade or a specialist tool.
- Apply a decking cleaner and let it dwell.
- Brush along the grain, never across it.
- Rinse with a hose, at normal pressure.
- Let it dry fully, then re-oil or re-stain if the water no longer beads on the surface.
What will ruin it
Decking is lethal when it is wet and green. Algae on timber is as slippery as ice, and a fall on decking steps is a genuinely common injury. Treat it before winter rather than after.
Why it works
Pressure washing tears the softer springwood out from between the harder grain lines, leaving a rough surface with far more area for algae and dirt to colonise. It gets dirty faster every time it is blasted.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.