What is actually greying the deck
Three different things make a deck look tired, and they want different answers. Get the diagnosis wrong and you will spend an afternoon fighting the wrong problem.
- UV-degraded lignin. The grey surface layer on any deck left unfinished. It is dead wood fibre and it lifts with a cleaner and gentle agitation.
- Mildew and algae. Black or green, worse where the deck stays damp. Biological, and it needs killing rather than removing.
- Tannin bleed and rust. Brown or orange streaks, often around fixings or under planters. Neither is dirt; both want an acid brightener.
Percarbonate, and why it wins on bare wood
Sodium percarbonate breaks down in water into hydrogen peroxide and soda ash. The peroxide oxidises the grey lignin and the mildew, the soda ash lifts the surface film, and neither of them attacks the wood fibre the way hypochlorite does. On bare timber it is the default.
Mix it warm if you can — it dissolves reluctantly in cold water and undissolved powder does nothing. Apply, keep it wet for ten to fifteen minutes, agitate with a soft brush along the grain where the grey is stubborn, and rinse at 500–1,200 PSI with a 40-degree white tip held well back.
Surface damage risk
When hypochlorite still has a place
Hypochlorite kills mildew faster than percarbonate and costs less. It also darkens some species, raises the grain, and will strip colour from a stained deck unevenly. The honest rule: use it on painted or heavily sealed decks where the coating is intact and doing the protecting, and on composite where the manufacturer permits it. Keep it off bare cedar, redwood and anything the customer is precious about.
| Deck | First choice | Avoid |
|---|---|---|
| Bare cedar or redwood | Percarbonate | Hypochlorite — darkens |
| Pressure-treated pine | Percarbonate | High pressure — furs badly |
| Composite | Manufacturer-approved cleaner | Anything above the maker’s max pressure |
| Painted or solid-stained | Mild detergent, dilute SH | Percarbonate at strength — can lift coating |
Brightening is a second step
An alkaline cleaner leaves the wood chemically alkaline, which darkens some species and interferes with the stain that goes on afterwards. An oxalic-acid brightener neutralises that, lifts tannin and rust streaking, and returns the wood to something close to its natural colour. It is fifteen minutes of work and it is what separates a cleaned deck from a restored one.
Charge for it as its own line. Customers who see the difference on half the deck rarely say no.
PSI & nozzle quick reference
- 0 degRed
- Spot jet. Etches most surfaces.4000 PSI
- 15 degYellow
- Stripping bare concrete.4000 PSI
- 25 degGreen
- General concrete and walkways.3500 PSI
- 40 degWhite
- Siding, wood, vehicles.1500 PSI
- ~65 degBlack
- Detergent draw, low pressure.500 PSI
Rinse, dry, and the stain conversation
Rinse thoroughly — residue left in the grain will interfere with any coating and can bloom back out as a white haze. Then tell the customer the part everyone skips: the deck has to dry properly before a stain goes on, and that means days rather than hours in anything but hot dry weather.
- 48 hours minimum in warm dry conditions, longer for pressure-treated timber which holds water.
- Test with a few drops of water: if they bead, the wood is not ready.
- Book the staining as a separate visit. It is a better job and a second invoice.
Composite is not wood and does not want any of this