Lap marks are one of the most common frustrations when oiling a deck, but they're almost always preventable. The secret lies in good preparation, working quickly across the full width of each board, and never letting a wet edge dry before you join the next stroke. Follow the steps below and you'll end up with a smooth, professional finish that protects your deck through a New Zealand summer and beyond.
Why Lap Marks Happen
Lap marks occur when a section of oil begins to dry or soak in before you overlap it with the next application. The overlapping area gets a double coat, leaving a visible ridge or darker stripe. Timber species, board width, temperature, and direct sunlight all affect how quickly oil absorbs — so understanding your conditions before you start is half the battle.
Preparation Is Everything
No amount of good technique will save a poorly prepared surface. Before you open the tin:
- Clean the deck thoroughly. Remove dirt, mould, mildew, and any old flaking finish. A deck wash or oxalic-acid-based cleaner works well. Rinse and allow the timber to dry completely — at least 48 hours in fine weather.
- Sand if needed. Rough or weathered boards benefit from a light sand (80–120 grit) along the grain. This opens the timber pores and helps oil penetrate evenly. Always sand with the grain, never across it.
- Check moisture content. Timber should ideally be below 18% moisture content before oiling. If the deck feels damp or was recently rained on, wait longer. Applying oil to wet timber is a leading cause of uneven absorption.
- Choose the right day. Avoid oiling in direct, harsh sunlight or when temperatures are above about 30°C. Early morning or a lightly overcast day is ideal. Check the product's recommended temperature range — most NZ decking oils specify between 10°C and 30°C.
- Mask off adjacent surfaces. Protect house walls, glass, and garden beds with masking tape and drop sheets.
Choosing the Right Product
NZ hardware stores stock a good range of decking oils suited to our climate. Brands such as Resene, Dulux, and Wattyl all offer penetrating decking oils formulated for hardwoods and softwoods common in NZ (pine, kwila/merbau, macrocarpa). Always read the datasheet for your specific product — application rates, recoat times, and thinning recommendations vary.
For new or very dry timber, some manufacturers recommend thinning the first coat slightly (typically 5–10% with the appropriate solvent) to aid penetration and reduce the risk of surface pooling.
The Right Tools
| Tool | Best for |
|---|---|
| Flat pad applicator | Wide boards, fast coverage, minimal lap marks |
| Short-pile roller (10 mm) | Large open areas, follow with a brush to work oil in |
| Natural-bristle brush | Narrow boards, between-board gaps, detailed work |
| Lambswool applicator | Smooth, even spread on hardwoods |
A flat pad applicator is generally the best choice for avoiding lap marks because it applies a consistent, thin film and lets you maintain a wet edge easily. Avoid foam rollers — they can create bubbles and uneven coverage.
Applying the Oil — Step by Step
1. Work board by board, full length
This is the single most important rule. Always oil one or two full boards from one end of the deck to the other before moving to the next board. Never stop halfway along a board — if you need a break, finish the full run first.
2. Maintain a wet edge
Work at a pace that keeps the leading edge of your application wet. If you're working in warm conditions, have a second person follow behind you to keep up, or work in smaller sections of the deck at a time.
3. Apply thin coats
Thick coats don't penetrate better — they sit on the surface, dry unevenly, and peel. Apply a thin, even coat and allow it to soak in. Most decking oils recommend wiping off any excess that hasn't absorbed within 15–30 minutes (check your product's instructions). Pooled oil left to dry is a guaranteed lap mark.
4. Work in the shade where possible
If part of your deck is in sun and part in shade, start in the sun while it's still manageable (early morning) and work toward the shaded area. Direct sun accelerates drying and makes maintaining a wet edge much harder.
5. Apply a second coat at the right time
Most products recommend a second coat within a specific window — often 2–6 hours after the first, while the first coat is still slightly tacky. Applying within this window helps the coats bond and reduces the risk of lap marks between applications. Check your product's datasheet for the exact recoat window.
Common Mistakes to Avoid
- Stopping mid-board — always finish the full length before taking a break.
- Applying in direct midday sun — the oil dries too fast to blend properly.
- Over-applying — more oil does not mean better protection; it means sticky, uneven surfaces.
- Skipping the wipe-back — excess oil that pools in grain or between boards must be wiped off before it skins over.
- Using old or separated oil — stir the tin thoroughly before and during use.
When to Call a Professional
Most homeowners can oil their own deck safely — it's not restricted work under NZ law. However, consider hiring a professional painter if:
- Your deck is large, multi-level, or requires working at height (follow WorkSafe NZ guidelines for working safely at elevation).
- The timber has significant damage, rot, or requires specialist preparation.
- You want a guaranteed, consistent result on a high-visibility area.
A qualified painter will have the right equipment and experience to get the job done efficiently. You can find local, vetted painters on tradedirectory.co.nz.
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