Deck oil is cheap and the job looks simple, which is why it is done badly so often. The oil is rarely the problem. The deck underneath it usually is.
When does it need it
Two tests, both quick.
The water test. Flick water onto the boards in a few different spots. If it beads, the finish is intact and re-oiling achieves nothing — fresh oil on a sealed surface sits on top, stays tacky, and collects dirt. If it soaks in and darkens the timber, the oil has gone.
Look at the colour. Merbau greys from the surface down under UV. Silvering means the finish has failed and the top fibres are breaking down.
Test in several places. A north or west-facing section, or the area that gets foot traffic, will need it well before a shaded corner does. Partial re-oiling is perfectly reasonable and better than doing nothing because the whole deck is not ready.
Typical intervals in Sydney: a deck in full sun wants doing every 12 months, a sheltered one every 18 to 24.
What has to happen first
In order, and none of it is optional.
Clean it. Oil applied over mould, grime or grey surface fibres bonds to that layer, and that layer is not attached to anything. It peels or goes blotchy within a season. Use a purpose-made deck cleaner rather than a pressure washer — see Merbau decking: cleaning, tannin bleed and re-oiling.
Brighten, if it has greyed. An oxalic acid deck brightener neutralises the cleaner and restores colour. It also releases tannin, so protect what is below the deck.
Rinse thoroughly. Residual cleaner or brightener left in the timber interferes with oil penetration.
Then wait. This is the step that gets skipped and it matters most.
The drying time
Merbau is dense and it holds water. After a wash, the surface looks dry within a few hours and the timber is not.
Oil applied to damp Merbau cannot penetrate, because the pores are full of water. It sits on the surface, stays sticky for weeks, goes patchy, and traps the moisture underneath where it feeds mould.
Allow two to three genuinely dry days in warm weather, and longer in winter or after rain. The test is a moisture meter if you have one, or simply: if the timber is still darker than you know it to be when dry, it is not ready.
This is the main reason to do deck work in autumn or spring rather than mid-winter. Sydney winter humidity makes a three-day dry window hard to come by.
Applying it
Thin coats. Merbau is dense and will not absorb much. The instinct to put it on thick produces a surface film that never fully cures and goes sticky in summer heat.
Two thin coats beat one heavy one, with the second going on once the first has absorbed — usually a few hours, not days.
Work board by board, full length. Stopping mid-board leaves a lap mark that will be visible permanently.
Wipe off anything that has not absorbed after 15 to 20 minutes. Oil pooling in the grooves or around fixings is oil that will stay tacky.
Not in direct sun. Hot timber flashes the solvent off before the oil penetrates. Early morning or late afternoon, or follow the shade.
Not with rain forecast within 24 to 48 hours.
Oil versus stain versus decking paint
Penetrating oil soaks in, wears away gradually, and re-coats with a clean and another application. It never peels. For Merbau this is almost always the right answer.
Film-forming decking stains and coatings last longer but fail differently — they peel, and stripping a failed film from a grooved deck is miserable work.
The choice is really between frequent easy maintenance and infrequent difficult maintenance. On a hardwood deck in Sydney sun, oil wins.
Rags and heat
Worth saying plainly because it is a real risk and it is widely ignored: rags soaked in drying oil can spontaneously combust. The oxidation that cures the oil generates heat, and a pile of oily rags in a bin or a corner of the garage holds that heat.
Lay them out flat to dry completely in the open, or soak them in water in a sealed metal container before disposal. Do not screw them up and leave them in a bag.
If the deck is past oiling
Oil restores colour and protection to sound timber. It does not fix splitting, cupping, loose boards or rot.
If the boards are furry from previous pressure washing, or deeply weathered, the deck needs sanding before any finish goes on. That is covered in timber decks: when to clean, when to sand, and when to leave it alone.
For the preparation side done properly, see deck cleaning.
