Why is the varnish peeling or failing on my boat?

Boat varnish usually fails because water gets underneath it, sunlight weakens the protective film, or the coating never bonded properly to the wood or previous coat. Check where the failure starts before sanding: lifting around screws and joints points to moisture, while widespread flaking or bubbles suggests preparation or application trouble.

Read the damage

Gently lift a loose edge with a scraper. Examine the exposed wood and the underside of the flake; the layer that separates shows where the bond failed.

• Bare wood beneath a clear flake: suspect damp timber, surface contamination, oily teak or poorly prepared weathered wood.
• Varnish remaining beneath the flake: suspect poor adhesion between coats, often caused by inadequate sanding, contamination or a missed recoating window.
• Milky patches or dark wood around fittings: check for water entering through screw holes, joints, exposed end grain or the back of the board.
• Cracking and peeling on exposed upper faces: prolonged sunlight and overdue maintenance may have left the film too thin or brittle.
• Bubbles appearing during application: suspect air escaping from warming timber, overworking the brush or applying too thick a coat.

Freshly stripped teak or mahogany often looks darker after varnishing than an old, sun-bleached surface. Colour change alone does not mean the coating is failing.

Stop water and remove loose varnish

Inspect companionway board edges, handrail feet, plugged screw holes and trim joints first. Even a perfect top surface will peel if water enters through an unsealed underside. Repair the entry point and let the timber dry before coating. Varnish cannot reliably seal moisture inside wood.

Use a moisture meter if available and follow the varnish maker's timber-moisture limit. Many systems require readings below roughly 15%, though the wood species, meter settings and product requirements affect the reading you need. A surface that looks dry just after rain proves little.

A small dent or isolated chip can usually be patched if the surrounding varnish is firmly attached. Remove loose finish, expose clean wood within the damaged area and feather the sound edges. Expect some colour difference at first.

Strip the whole face if peeling is widespread, cracks run throughout the finish, or lifting continues as you scrape. A sharp cabinet scraper gives better control than grinding away thick varnish with coarse paper. Take particular care on veneered hatch boards: the decorative veneer can be very thin.