Timber rots fastest not from one soaking but from being wetted and dried over and over while it sits unprotected. Right now the beds are empty and summer growth has died back, so you can get a brush to every face of a fence, gate or raised bed, including the end grain of posts, which drinks up moisture fastest and rots first. Preservative applied over damp wood just seals the damp in, so this is a job for the next dry spell, not the first free afternoon.

Treat Fence Panels, Gates and Raised Bed Timber Before Winter

What you need

2½ litres exterior wood preservative or stain for fences, gates and the outer faces of raised beds; covers roughly 8–12 square metres per coat on bare wood
1 litre raw linseed oil or tung oil for raised bed faces that touch soil you grow food in — not boiled linseed oil, which contains metal driers
1 pair disposable or nitrile gloves
1 dust sheet or old bedsheet to catch drips off paving and nearby plants
½ litre white spirit only needed if you use an oil-based preservative; water-based ones clean off brushes with water
1 handful clean rags

Step by step

Check the forecast and pick a dry day: above about 9°C, with no rain due for the next 24 hours.
5 min
Cut back or tie off any plants growing against the timber and clear debris from the base, so you can reach every face.
20 min
Scrub off dirt, algae and any flaking old coating with a wire brush, working back to sound, bare wood. Don't paint over flaking paint — it just traps moisture underneath.
30 min
Lay the dust sheet underneath, and if the wood is damp from recent rain, leave it a full dry day before going any further.
10 min
Stir the preservative well and apply a thin first coat with the grain, working top to bottom and finishing the end grain of every post, which is the most porous part and rots first.
40 min
On raised bed faces that touch soil, use linseed or tung oil instead and work it in the same way.
20 min
Let the first coat dry to the touch, per the tin, then apply a second coat for full protection.
20 min
Clean brushes immediately — water for water-based preservative, white spirit for oil-based — so the bristles don't harden.
5 min

Afterwards

Lay oily or linseed-soaked rags flat outside until completely dry, or soak them in water, before binning them — never leave them bunched up. Keep the lid tight on leftover preservative and store it somewhere frost-free. Before planting in a treated raised bed, wait for a full cure: water-based preservatives are usually fine after 24–48 hours, but raw linseed oil can stay tacky for a week or more in cool weather, so judge it by smell and touch rather than the calendar. Check the whole run again after a couple of seasons for soft, spongy wood, which needs replacing rather than another coat.

Questions

Can I use creosote?

No. Creosote, and old CCA or penta-treated timber, aren't suitable near anything you intend to eat, and creosote sale is now restricted to agricultural and industrial use. Use a proper exterior wood preservative or stain instead.

How long before I can plant in a treated raised bed?

Follow the tin. Water-based preservatives are usually safe after 24–48 hours; raw linseed oil can take several days to over a week to stop feeling tacky. Wait until it's fully dry and odourless, not just dry to a quick touch.

Do I need to treat timber that's already pressure-treated or tanalised?

No, not for rot — pressure treatment carries preservative all the way through the wood. A stain on top is just for colour, and can wait a few years.

What if I only have time to do one thing?

Do the end grain of posts and corners first. That's where water sits and soaks in, and where rot starts, even if the flat faces never get a second coat.