How do you make wood water-resistant?
Tips & maintenance · Maintenance
MaintenanceMaking wood water-resistant
You can't make wood waterproof. You can treat wood so that water hardly soaks in and can get out again — and that is exactly what you want.
- Waterproof doesn't exist
- End grain absorbs 10× faster
- Breathable beats sealed
Why sealing doesn't work
A layer that keeps water out also keeps water in. And that is usually the bigger problem.
Wood absorbs moisture from the air and releases it again; that is not a flaw but the nature of the material. A dense, impenetrable layer all round a wooden object seems the answer, until it gets damaged somewhere. Then water gets in that can no longer get out, and the wood rots under a layer that still looks perfect from the outside.
What does work is slowing down absorption and keeping drying possible. Oil penetrates the fibre and lowers the rate of absorption without completely sealing the pores. A good stain does the same and adds UV protection. Impregnation makes the wood unattractive to fungi, but doesn't keep water out.
And then there's the shape. The best moisture protection is a design in which water cannot stand: a fall on horizontal surfaces, rounded top edges, a capping strip over end grain, and room to dry everywhere. That makes more difference than any product.
End grain is the open ends of thousands of wood vessels. Water soaks in there up to ten times faster than through the side. Two extra coats on every end grain do more for the lifespan than an extra coat over the whole surface.
- Waterproof
- doesn't exist with wood
- Oil
- slows absorption, lets the wood breathe
- Stain
- like oil, plus UV protection
- Lacquer or varnish
- seals — fine indoors, risky outdoors
- Impregnation
- against fungi, not against water
- End grain
- two extra coats
- Design
- fall, rounded edges, room to dry
To be fair: every treatment is temporary. Oil lasts one to two years outdoors, stain two to five, lacquer longer but with the risk that it cracks and traps water. There is no one-off treatment that protects wood for ever — if that is what you are after, you are better off choosing a species that can naturally cope.
A layer that keeps water out also keeps it in
A dense layer seems the answer, until it gets damaged once. Then water gets in that can no longer get out, and the wood rots under a layer that looks perfect from the outside. Oil penetrates the fibre and slows absorption without sealing the pores.
Click a chapter or use the arrows · moisture absorption schematic.
What each product does
| Product | What it does | How long | Where |
|---|---|---|---|
| Wood oil | penetrates, slows absorption, lets it breathe | 1 – 2 years outdoors | garden furniture, decking, hardwood |
| Stain | like oil plus UV protection and colour | 2 – 5 years | cladding, fences, joinery |
| Lacquer or varnish | seals, hard surface | long, until it cracks | indoor work; outdoors only when sheltered |
| Pressure impregnation | against fungi and insects | the lifespan of the wood | posts, garden timber, construction |
| Wax or hardwax oil | nourishes and protects the surface | 1 – 3 years indoors | indoor furniture and tops |
| Water-repellent impregnating coating | greatly reduces absorption | 1 – 3 years | cladding and stone-to-wood junctions |
How to go about it
- CleanRemove dirt and green algae with a soft brush and soapy water. Any product on a dirty surface only half adheres.
- Let it dry completelyAt least 48 hours of dry weather. Treating damp wood traps the moisture — exactly what you want to avoid.
- Sand lightlyGrit 120 to 180 where the surface is rough or grey. That opens the fibre so the product soaks in.
- End grain firstTreat every end grain separately and twice, before you start on the faces. They absorb so much that you can see the difference.
- Apply thinlyTwo thin coats are better than one thick one. Wipe off excess oil after a quarter of an hour — what stays on becomes sticky.
- Don't forget the undersidesThe underside of a tabletop, the bottom of a door, the back of a cladding board: those are the surfaces where it always starts.
- Check every yearWater that keeps beading on the surface means the coat is still working. If it soaks in straight away, it is time.
Designing water away
A fall on horizontal surfaces
A sill or top rail with a slope of a few degrees drains water instead of letting it stand. Two degrees is enough.
Rounded top edges
Water doesn't cling to a rounded edge, and paint or oil adheres better there than on a sharp arris.
A capping strip over end grain
On fences, posts and cladding. The simplest detail with the biggest effect.
Everything clear of the ground
At least a few centimetres of air under every wooden element. Splashing water and permanent damp are the main causes of rot.
Ventilation behind the wood
A façade or a planter must be able to dry at the back. Without that, no treatment will help.
Space between parts
A few millimetres between decking boards or fence boards lets water through and air to the wood.
Frequently asked questions about making wood water-resistant
Can wood really be made waterproof?
No. You can greatly slow down water absorption, but completely sealing a wooden object is not a good idea: at the first damage, moisture gets in that can no longer get out.
Oil or lacquer for outdoors?
Oil. It lets the wood breathe and can be touched up locally. Lacquer seals and eventually cracks, after which water gets underneath.
Why is the end grain so important?
That is where the wood vessels are open. Water soaks in there up to ten times faster than through the sides — which is why rot usually starts on an end-grain surface.
How often do I need to treat it?
Wood oil outdoors every one to two years, stain every two to five years. Test with a drop of water: if it beads, the coat is still working.
Does impregnation protect against water?
Not really. Impregnation protects against fungi and insects; the wood still absorbs and releases moisture, and still turns grey.
What does more: treatment or good design?
Design. A fall, rounded edges, everything clear of the ground and room to dry do more for the lifespan than any product.
More on maintenance
Which oil, which stain, and what each wood species needs.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Water droplets beading on weathered wood with shrinkage cracks — Brossmjo, CC BY-SA 4.0 · Beilstein, Germany — Roman Eisele, CC BY-SA 4.0