Cracked and Flaking Varnish on a Front Door

Why does varnish crack and alligator on an exterior door?

Clear finishes have almost no pigment, so UV goes straight through the film and attacks two things at once. It embrittles the film itself, so it can no longer flex, and it degrades the lignin at the wood surface underneath, which begins photodegrading within hours of UV exposure (USDA Wood Handbook chapter 16).

Meanwhile the wood keeps moving. Exterior wood in this area swings between about 11.1 and 12.6 percent moisture content across the year (Wood Handbook Table 13-1), swelling and shrinking under a film that has lost its elasticity. Add roughly 90 freeze-thaw cycles a year, derived from NOAA daily records, 1991 to 2020, and about 118 separate rain events (NOAA NCEI climate normals).

The film cannot follow the wood, so it fractures. First hairline crazing, then a distinct alligator pattern, then flakes lifting at the edges once water gets into those cracks and works underneath.

How do I know if the cracks have reached the wood?

Pop off a loose flake and look at its back. This is the single most useful test on a clear-coated door, and it comes straight out of the Forest Products Laboratory's description of how clear varnish fails: by a combination of cracking and UV degradation of the wood at the wood-varnish interface, identified by finding wood fiber attached to the back of a chip.

Wood fiber on the back means the failure is at the wood surface. Everything has to come off, back to fresh wood, because the layer your next coat would bond to is already destroyed.

Clean film on the back, with the door still showing its color and no gray, means the cracking is confined to the coating and you caught it early.

Also look into the cracks with a flashlight. If you can see gray or silver down in them, UV has reached the wood, whatever the flakes tell you.

Why did my varnish only last a year or two?

Because that is what clear finishes do in direct sun, and it is worth seeing this in writing before anyone sells you a miracle product. The Wood Handbook states that varnish lacks exterior permanence unless protected from direct sunlight, and that varnishes in direct sunlight generally require refinishing every 1 to 2 years. The same chapter ranks exterior protection from least to most as clear varnish, pigmented varnish, semitransparent stain, solid stain, then paint. Clear is at the bottom of the list.

Orientation decides where in that range you land. A north-facing finish lasts about twice as long as a south-facing one, and the Park Service notes that south and west elevations weather fastest.

There is a local twist too. A south-facing door here takes more direct sun in November through February than in June and July (NASA POWER, 2001 to 2020), because the low winter sun strikes a vertical door face nearly square on.

What happens if I just scuff and recoat over the cracks?

If the cracks are only in the film and the wood underneath is untouched, scuffing and recoating is legitimate maintenance and we do it regularly. That is the whole point of catching it early.

If the cracks have reached the wood, recoating fails and it fails fast. The new coat flows into the cracks, cures, and is now mechanically locked to a layer that is already delaminating. When that layer lets go the new coat comes with it, usually within a season or two, and often in bigger sheets than before.

There is a second problem with repeated recoating: film build. The Park Service notes in Preservation Brief 10 that excessive thickness is enough on its own to trigger cracking and peeling, and that thick films fail at the weakest point of adhesion, which is the oldest layers next to the wood. Every coat you add over a failing film makes the eventual strip harder and more expensive.

Can I strip and revarnish a door myself?

It is one of the harder finishes to do well, and here is why. Alligatored varnish sits in cracks and in the corners of panel profiles, and it does not come out with a sander. It has to be softened with chemical stripper and worked out of the detail with picks and brushes, over multiple applications, without gouging the wood.

Then the coating schedule is unforgiving. The Wood Handbook specifies a minimum of three coats for exterior varnish even in protected areas, and moisture-exclusion testing shows why: one coat blocks about 2 percent of moisture over two weeks, two coats 23 percent, three coats 44 percent (Table 16-3). Skimp on coats and it fails early no matter how good the prep was.

Every coat also has temperature and humidity conditions that must hold through the whole cure, not just at brush time. A weekend project turns into three weekends with a door standing bare in between. And never power wash it off. Preservation Brief 47 warns that drives water into the wood and the wall.

What does it cost to strip and rebuild a failed clear coat?

A single entry door is $550 to $1,400. Double doors and French doors are $950 to $2,200. A wood garage door is $1,200 to $3,200. Heavily alligatored clear coats sit toward the upper half of those bands because stripper has to be applied several times to lift varnish out of the profiles and the cracks.

Caught early, before the cracks reach the wood, a maintenance recoat is a much smaller job and a much smaller number. That is the whole argument for putting a south or west facing door on a schedule rather than waiting for it to look bad.

A replacement door is $3,500 to $8,000 or more installed. Replacing a sound door because its clear coat cracked is buying new wood to solve a coating problem.

Text a photo of the cracked area, close enough to see the pattern, to (202) 979-9977.

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