Why is my deck stain peeling?

Deck stain peels when it has formed a film on top of the wood instead of soaking into it. A film can crack, let water in behind it, and lift; a stain that has soaked in has nothing to lift. The USDA Forest Products Laboratory's Wood Handbook puts it simply: "Finishes that do not form a film do not peel".^[1] So the question is why your finish formed a film, and the answer is usually one of the causes below.

The usual causes

A film-forming product on a deck. Decks are hard on any coating. In the handbook's words: "Horizontal surfaces exposed to the rain and sun, such as decks, are prone to peeling."^[1] Paint and solid-color stain are films by design: "Solid-color stains form films and so will be prone to peeling just like paint."^[1] The handbook adds that solid-color stains lack abrasion resistance and manufacturers do not generally recommend them for decks.^[1]

A semitransparent stain that was not really penetrating. The handbook explains that waterborne semitransparent stains often get their look by forming a thin coating on the wood, and so can fail by peeling and flaking.^[1] It adds that waterborne semitransparent stains with high solids content are especially prone to forming a film.^[1] If the label said semitransparent but the deck had a sheen when it dried, this is likely your cause.

Too much stain. Even a penetrating stain forms a film if the wood cannot absorb it all, and a second coat may form a film even if the first did not.^[1] If the finish still looks shiny an hour after it goes on, it has not soaked in,^[1] and thin films like that may crack and peel within a year or two.^[1] Rollers and sprayers make this easy to do on a flat deck.

Finish over weathered wood. Sunlight breaks down the surface of bare wood quickly. The handbook found that a film-forming coating applied to wood that has sat in the sun for about a week (low-density woods) to four weeks (high-density woods) is prone to peeling.^[1] Paint applied to weathered wood often fails over large areas, and sometimes wood fibers are attached to the back of the film,^[1] so look at the back of a loose flake. Grain on its back points to this cause.

Wet wood. New pressure-treated lumber is sometimes still soaking wet when the deck is built, and nothing will coat wet wood well.^[1] Films are also prone to blister and peel if the wood underneath stays wet,^[1] and persistent moisture in the wood is one reason film-forming finishes do so badly on horizontal surfaces such as decks.^[1]

Board ends. Board ends take in water far faster than the sides,^[1] so they are the hardest place on a deck to keep protected, and worth checking first when you look for the cause.

Incompatible layers. "Do not apply oil-based semitransparent stains over film-forming finishes."^[1] A penetrating stain put over an old film sits on the film, not in the wood, and comes off with it.

How to fix it

There is no coat you can put on top that stops peeling. The handbook explains why patching rarely works: "Nothing will penetrate areas having a film; film-forming finishes (paint or solid-color stain) do not bond to weathered areas."^[1] A deck that is part peeling film and part bare, weathered wood has two surfaces that want different finishes.

You have two choices. In the handbook's words, either remove the finish where there is a film and re-stain, or sand the weathered areas, scrape and sand the filmed areas, and refinish with solid-color stain or paint.^[1]

• Back to a penetrating stain. More work now, much less later: once the film is gone, future recoats are mostly cleaning and one thin coat. See how to stain a deck.
• Stay with a film. Less stripping now, but expect to scrape, sand and recoat the worn spots on a regular cycle, because a film on a deck tends to keep failing.

Stripping is the slow part. The handbook warns that products that are effective at stripping paint tend to pose health risks, that chemical fumes can be toxic, and that you need effective safety equipment including a respirator, because the dust masks sold in hardware stores do not block chemical fumes.^[1] Follow the product label, and whatever method is used, sand the wood before applying the new finish.^[1]

Older decks need more care. If your deck was built before 2004 and is not cedar or redwood, it was most likely built with CCA-treated wood,^[2] and the U.S. Consumer Product Safety Commission advises you to avoid sanding or power washing CCA-treated wood.^[2] If the house was built before 1978 and there is old paint involved, the EPA notes that any renovation, repair or painting project in a pre-1978 home that has lead-based paint can easily create dangerous lead dust.^[3] In either case, get advice before you scrape or sand.

To keep it from happening again, see how often a deck should be stained or sealed.

Stop and call a pro if

• The wood under the peeling finish is soft, rotted or badly split, or railings and stairs are loose. Fairfax County's building official advises that when the connections or the wood look wrong, consider staying off your deck until a professional inspects it.^[4]
• The deck was built before 2004 and the fix would mean sanding or power washing.
• The house was built before 1978 and the peeling finish may be old paint.
• The whole deck needs chemical stripping and you do not want to handle the stripper.

Getting it fixed on Loclis

On Loclis you describe the deck staining job once, with photos of the peeling, and local businesses quote it. See also what deck staining costs, browse deck staining and sealing, find exterior pros around Blacksburg, or read more exterior answers.