Why did my exterior varnish flake and peel off?

Clear finishes outdoors fail from underneath, because UV destroys the wood surface through the film. Why varnish is a demanding choice outside, and what to do instead.

Difficulty: intermediate
Time: 2 hr
Read: 5 min
Safety: caution

Short answer

Ultraviolet light passes through a clear film and degrades the timber surface beneath it, so the varnish loses its grip and lifts — usually starting on the sunniest elevation and at every joint. Strip it, and either commit to annual maintenance with a UV-filtering exterior varnish, or switch to a pigmented stain or paint.

A varnished front door looks superb for two years and then starts shedding. That is not usually poor workmanship. A clear finish is transparent to the ultraviolet light that destroys the wood fibres immediately below it, and once those fibres have broken down the film has nothing sound to hold onto. Pigment is what stops UV, which is why opaque and semi-transparent finishes last so much longer outdoors than clear ones.

Safety

Stripping exterior joinery often means working at height and dealing with old coatings. Paint and varnish applied before the 1990s may contain lead and must not be dry-sanded or burnt off. Use a tower or scaffold rather than reaching from a ladder, and wet-sand or use a chemical stripper on old finishes.

What you'll need

A chemical stripper or a scraper and wet abrasive
An exterior wood varnish with UV absorbers, or a pigmented alternative
Wood preservative and a suitable primer or basecoat
A moisture meter, to check the timber is dry before recoating
Safe access for anything above ground-floor level

Step by step

Confirm the failure mode.UV failure looks like flaking that starts on the sunniest face, often with the timber underneath visibly greyed and fibrous. Water failure starts at the bottom rail and the joints and shows as blistering. Most failed exterior varnish is both at once, in that order.
Accept that partial repair rarely works on a clear finish.The degraded wood surface extends beyond the visible flaking. Feathering into sound varnish leaves a boundary that fails again within a season. On a door or a window, strip the whole element rather than patching it.
Strip back to bare timber.A chemical stripper suited to varnish, or scraping and wet abrasion. On old joinery, avoid dry sanding and heat guns because of the lead risk. Get every trace of the old film out of the mouldings and the rebates, because it is what will show later.
Sand the degraded grey layer away.The greyed, fibrous surface left after stripping is UV-damaged wood, and nothing adheres to it properly. Sand back until you reach sound, evenly coloured timber. This is the step that decides how long the new finish lasts.
Let the timber dry and treat it.Check with a moisture meter if you have one, or simply give it several dry days. Apply a wood preservative to bare exterior joinery, particularly at the end grain and the joints, and let it soak in.
Choose the finish knowing the maintenance it demands.A clear exterior varnish with UV absorbers, applied in the number of coats the maker specifies and topped up annually before it starts to fail, is a genuine option — but it is an annual commitment. A semi-transparent pigmented stain lasts substantially longer and still shows the grain. Opaque paint lasts longest of all.
Build enough coats and cover the end grain.Exterior varnish systems typically want more coats than interior ones, and the end grain and joints want extra. Thin the first coat as the maker directs so it penetrates rather than sitting on top.
Maintain by recoating before it fails, not after.The whole economics of a clear exterior finish depend on this. A light abrade and one fresh coat each year on the exposed elevations costs an afternoon; letting it break down costs a full strip. Inspect every spring.

Tips

A south or west-facing door gets several times the UV of a sheltered north-facing one. The same finish on the same house can last four times as long on one elevation as another.
A porch, a canopy or a deep reveal over a varnished door does more for its life than any product choice.
If you love the look of varnished timber but not the maintenance, a semi-transparent pigmented stain is the compromise that actually works — enough pigment to block UV, enough transparency to see the grain.

Common mistakes

Using yacht varnish because it sounds tough — Many yacht varnishes are formulated for flexibility rather than UV resistance, and some contain little UV protection at all. Use an exterior wood varnish specifically formulated with UV absorbers.
Recoating over flaking areas without stripping — The wood surface under and around the failure is UV-degraded. The new coat bonds to damaged fibres and lifts within a season.
Waiting until the finish is obviously failing before recoating — By then the timber beneath has degraded and a full strip is required. An annual maintenance coat prevents that entirely.

If it doesn't work

Flaking concentrated on the sunniest face

Cause: UV degradation of the timber surface through the clear film — Fix: Strip that element completely, sand off the greyed layer, and rebuild with a UV-filtering exterior varnish or switch to a pigmented finish.

Milky or cloudy patches under the varnish

Cause: Moisture trapped between the film and the timber — Fix: Strip the affected area, find where water is entering — usually a joint or end grain — dry the timber, and recoat with heavy attention to the joints.

Blistering at the bottom rail while the rest is sound

Cause: End grain absorbing water — Fix: Strip and repair that area, treat and prime the end grain heavily, and check the sill drainage and the drip groove.

Keeping on top of it

Inspect varnished exterior joinery, especially the sunniest elevation (Every spring)
Light abrade and apply a maintenance coat to exposed elevations (Annually)
Full strip and rebuild (Only when maintenance has been missed and the film has broken down)

Questions people ask

Why does clear varnish fail so quickly outside?

Ultraviolet light passes through the clear film and destroys the wood fibres just beneath it. Once those fibres break down, the varnish has nothing sound to grip and lifts. Pigment is what blocks UV, which is why opaque finishes last far longer.

Is there an exterior varnish that lasts?

Exterior varnishes with UV absorbers last considerably longer than general-purpose ones, but none is maintenance-free. Expect a light abrade and a fresh coat annually on exposed elevations.

What should I use instead of varnish on a front door?

A semi-transparent pigmented exterior woodstain keeps the grain visible while blocking UV, and lasts several times longer. An opaque exterior paint system lasts longest of all.