A preserver is timber's medicine, not its cosmetics: a penetrating carrier delivering biocides — fungicides against the rot species (wet rot, dry rot) and mould, and insecticides against borers (woodworm and their regional relatives). It soaks into the outer wood and makes it inedible.

The division of labour with the products around it: stains and paints provide colour and UV defence, oils feed and water-repel, but none of the decorative family kills organisms — while most pure preservers, conversely, do nothing for UV grey or looks. They are layers, not rivals: preserver first as the biological base, decoration over it as the weather skin. Many modern products bundle both ('preserver + stain', 'all-in-one shed & fence treatment'), trading each function's maximum strength for one-coat convenience — legitimate for fences, less so for critical timbers.

Where dedicated preserver is non-negotiable: cut ends of pressure-treated wood, ground-contact and shaded chronically-damp timber, repair work around previous rot or insect damage, and any bare untreated softwood heading outdoors. It is the cheapest insurance in the timber-care aisle, precisely because it is the one you cannot see working.

Mostly no on their intact faces — decisively yes at every cut, notch, and bore.

What pressure treatment did: forced preservative deep into the outer band of the timber under vacuum/pressure. That envelope is genuinely durable — years to decades against rot and insects depending on treatment class — and re-brushing preserver onto intact treated surfaces adds little (brush penetration is millimetres against the factory's).

Where the envelope fails: everywhere you cut. Sawing a post to length, notching a joist, drilling a bolt hole exposes untreated core — and end grain, the very face that drinks water fastest, is usually the cut. The rule written into good practice (and many treatment warranties): liberally re-treat every field cut with a brush-on end-grain preserver before assembly. Post tops, rail ends, and drilled fixings on a fence are precisely where the first rot appears years later, and always at the cuts nobody painted.

Also worth preserver on treated wood: timber whose treatment class is light for its new duty (fence-rated boards pressed into ground contact), older stock whose surface has weathered open, and anything showing early grey-black staining in permanently damp detail.

Treated timber plus cut-end discipline is how landscape structures reach their advertised decades; treated timber minus it is how 15-year posts become 6-year posts, snapped at the bottom cut.

By carrier and by mission:

Solvent-based preservers penetrate deepest and re-treat weathered dense timber best; stronger smell, longer dry, classic choice for structural and remedial work. Water-based preservers dominate the general market: low odour, fast drying, easy cleanup, modern biocide packages — right for fences, sheds, and routine protection, especially where people, plants, and pets live nearby.

By mission:

1. General-purpose exterior preservers (fungicide + insecticide) — the default for bare softwood outdoors.
2. End-grain / cut-end treatments — high-load formulations for field cuts on treated timber.
3. Woodworm killers — insecticide-led remedial treatments for active infestation in furniture, floors, and roof timbers.
4. Rot-repair systems — preserver plus hardeners and fillers for consolidating locally-decayed windows and sills.
5. Preserver-plus-colour combos — fence-and-shed one-can products where convenience rules.

Choosing: match the label to the target (a fungicide-only product ignores beetles, and vice versa), check it is overpaintable if decoration follows (some oil-rich combos resist topcoats), and for anything near ponds, veg beds, or livestock, read the environmental panel — modern water-based products are formulated for exactly those neighbours, older-style heavy solvents are not. For load-bearing or infested structural timber, note that professional-only treatments and specialist survey exist above the retail tier — the DIY tin is for protection and minor remediation, not for the roof that already crunches.

Preserver only protects the fibres it reaches — application is a loading exercise:

1. Start bare and dry. Preserver cannot penetrate paint, varnish, intact oil, or waterlogged wood. Strip or weather old finishes; treat at moisture the tin states (typically <20%); brush off dirt and any friable grey surface.
2. Flood coats, not licked ones. Apply liberally by brush along the grain until the wood stops drinking — typically two to three coats wet-on-nearly-wet, per the product's schedule. Coverage figures on the tin assume this generosity; halving them halves the protection depth.
3. Load the vulnerable details: end grain until it refuses more, joints and overlaps, tops of horizontal members, the bottom 300 mm of anything near ground, and both faces where access allows — the unseen back of a fence board rots first.
4. Dipping beats brushing where practical: fence board ends and post bottoms stood in a bucket of preserver for minutes absorb what brushes never deliver.
5. Respect the intervals: between preserver coats (hours) and before overcoating with stain or paint (often 24–48 h — carrier must clear or the topcoat fisheyes).
6. Safety basics: gloves and eye protection, ventilation indoors, keep animals off until dry, and dispose of leftovers as the label says — these are biocides doing biocide work.

Renewal: bare-standing preserver re-treats every 2–4 years exposed; under sound decorative coats it lasts as long as the coat keeps water out — which is why the system, not either layer, is the protection.

It depends on which side of 'structural' the rot sits:

Early surface decay — yes, with the moisture fixed first. Rot is a symptom; water is the disease. Find and stop the source (failed cap, blocked drip, soil contact), let the timber dry below ~20%, cut or scrape back to sound bright wood, then saturate the area — generously past the visible damage — with a dedicated remedial preserver. The biocide kills remaining fungus in the fibres; the dryness keeps it dead. Small soft patches in windows and sills additionally take wood hardener (consolidates the weakened zone) and exterior filler — the standard joinery-repair sequence.

Established structural rot — no tin fixes it. Timber that probes soft through its section, crumbles, or carries the cuboidal cracking of advanced decay has lost strength that chemistry cannot return: those members need cutting out and replacing (splicing in treated timber, with the new cuts preserved, and the moisture fault corrected). Dry rot specifically — the strand-spreading species that travels through masonry — escalates further: it demands identification and a methodical eradication (removal beyond visible growth, masonry treatment, moisture control), professional territory in occupied buildings.

The honest triage: preserver is excellent at keeping wood sound and at sterilising minor, dried-out decay; it is a preservation product, not a resurrection one. The screwdriver test — firm wood everywhere within 100 mm of the damage — is the boundary between 'treat and fill' and 'cut and replace'.

Used as labelled, modern retail preservers coexist with gardens — but this is the product family where labels genuinely matter:

The reassuring baseline: today's water-based consumer preservers use targeted, low-mobility biocides at regulated concentrations, a world away from the banned old guard (creosote, CCA and their kin — which is also why reclaimed railway sleepers and old telegraph poles don't belong beside vegetables). Dry, cured preserver is the design point: the hazard window is the wet phase.

The practical rules:

1. During application: sheet over or wet-then-rinse adjacent planting; keep pets and children off until surfaces are fully dry (label times; typically 24–48 h); never spray on windy days near ponds — aquatic toxicity is the sharpest warning on most labels.
2. Vegetable beds: for new raised beds, use timber whose treatment is rated for ground contact and current food-garden guidance (modern copper-based treatments are widely accepted), and if adding brush-on preserver, choose one explicitly safe for horticultural use once dry — or line the bed's inner faces with a barrier membrane, the belt-and-braces standard.
3. Animal housing (hutches, coops, kennels, beehives): only products specifically labelled animal-safe when dry — chew-height timber makes ordinary formulations the wrong choice.
4. Ponds: treat nearby structures with aquatic-caution products in dry weather, cured well before rain.

Storage and disposal close the loop: sealed tins away from frost, and leftovers to hazardous-waste collection rather than drains. Respect the wet phase, choose garden-labelled chemistry at the sensitive spots, and preserved timber is what makes the garden's structures outlive its seasons.

Bare or preserver-only timber: every 2–4 years. Preserver under maintained decorative coats: when the coat itself needs renewing. Then adjust for the real variables:

Exposure grade: wind-driven-rain faces, coastal salt, and full-sun/hard-frost cycling push toward the 2-year end; sheltered courtyard fences drift toward 4+. Detail risk: ground-line, post bases, panel bottoms, and horizontal rails age fastest — many owners sensibly re-treat the bottom third annually and the whole structure on the slower cycle. Product class: thin clear preservers refresh more often than heavily-pigmented preserver-stains, whose pigments also carry UV duty.

Read the timber, not the diary: re-treat when water soaks in matte instead of beading, when surfaces look dry and bleached, or at the first grey-green biological film in damp corners. Renewal is easy caught early — clean, dry day, flood coat — and escalates to scraping and remedial work only when years slip.

The shed-specific notes: roofs count (felt laps and ridge timbers fail before walls); door bottoms and floor bearers are the shed's ground line; and the invisible back face against the neighbour's hedge is the classic omission — its first inspection is often the discovery of the whole structure's worst timber.

The economics stay lopsided in maintenance's favour: a fence panel is real money, an afternoon with a 5-litre tin is not — and the difference between the 10-year fence and the 25-year fence is almost entirely which years the tin came out.