Why is the paint on my window trim cracking?

Paint on window trim cracks when it can no longer move with the wood underneath it. That happens for a handful of reasons: old layers have built up until they are hard and brittle, water is getting into the wood at the sill, the joints or the end grain, the wood itself is checking, or the coating went on too thin or over weathered wood. The crack pattern usually tells you which, and the fix depends on it.

Read the crack pattern

A fine web of hairline cracks in the top layer. This is crazing. Crazing, fine jagged breaks in the top layer, appears when paint several layers thick becomes hard and brittle with age and can no longer expand and contract with the wood.^[1] It is common on older houses, because once paint builds up to about 1/16 inch, roughly 16 to 30 layers, one more coat can be enough to set off cracking and peeling.^[1]

Deep cracks in a grid, like reptile skin. This is crazing that was left too long. Left alone, crazing lets water in, the wood swells, and the cracks deepen into alligatoring, a pattern of horizontal and vertical breaks that looks like reptile skin.^[1]

Long cracks running with the grain. Cracking parallel to the grain shows up on smooth flat-grain lumber, often along the dense latewood bands, and is helped along by raised grain and by coatings that are too thin or not flexible enough.^[2]

Cracks running across the grain. Modern latex paints seldom crack across the grain.^[2] When they do, the wood under it is probably moving, such as checking juvenile wood cut from near the center of the log, which calls for replacing the board.^[2] Otherwise, cross-grain cracking usually turns up where there are thick layers of old oil-alkyd paint, and old alkyd paint potentially contains lead.^[2]

Cracks at miters, butt joints and finger joints. Separate pieces of wood move separately. Finger-jointed boards are common for exterior trim, and paint often fails at a joint where the pieces on either side move differently with moisture.^[2]

Cracks over knots. Knots are extremely prone to cracking,^[2] and even the best paints cannot bridge over large cracks or checks that develop in the wood.^[2]

Cracks with peeling or blisters, worst at the sill. That is water. Flaking often follows cracking; small cracks in paint allow water to enter.^[2]

Why window trim fails first

Windows combine everything paint dislikes. Sills face the sky, and exposed horizontal surfaces of window frames tend to deteriorate much faster than vertical ones.^[3] Casings have many joints and end grain where they meet the sill. Sun matters too: dark colors are more susceptible to water blisters than light ones because they get hotter in the sun.^[2] And water that gets behind the trim shows up as blisters and peeling, since water blisters are common near unsealed end grain of trim and where rain gets in around improperly flashed windows.^[2]

Check for rot before anything else

Decayed wood does not hold paint,^[2] so probe the sill and the bottom of each casing with a small screwdriver. The National Park Service notes that rot is a strong possibility on a sill that has sat under standing water, and the wood should be repaired or replaced before repainting.^[1] If it is soft, see how to replace rotted exterior trim.

Stop the water

Repainting before the water is fixed only buys a season. The usual sources are faulty flashing, leaking gutters, cracks and holes in siding and trim, deteriorated caulking in joints and seams, and shrubbery growing too close to painted wood.^[1] Clean out the gutters, cut plants back from the wall, and recaulk open vertical joints. For more on gutters, see why gutters overflow.

Fix the paint to match the failure

• Crazing or cracks in the top layers. If cracking is only in the top layers, scrape and sand to the next sound layer and repaint.^[1] For cracks along the grain, "If the cracking is not too severe, sand and apply one or two topcoats to give additional film-build."^[2]
• Alligatoring or cracks down to bare wood. If it has reached bare wood and the paint is flaking, the paint needs to be removed entirely, and bare wood should be primed within 48 hours.^[1]
• Not sure how well the old paint is bonded? Prime a small patch, let it dry overnight, press on an adhesive bandage and snap it off. If both the new paint and the old paint stick to the tape, the old paint is not well bonded to the wood and must come off.^[2]

When you prime, choose the primer with care: latex primers stay more flexible and are less likely to crack as they age than older oil-based primers.^[2] And do not repaint on a schedule for its own sake. Too many coats of latex paint can eventually lead to adhesion failure of the primer.^[2] "If paint is sound, but discolored with mildew, wash it. It does not need repainting."^[2]

On a house built before 1978, assume the old paint contains lead. Mist before scraping, use wet-dry sandpaper rather than dry sanding, and if paint must be softened, use a low-temperature heat gun under 1,100 degrees Fahrenheit or a chemical stripper, used by the label.^[4] If you hire the work out, contractors paid to disturb paint in a pre-1978 home must be EPA lead-safe certified.^[5]

Stop and call a pro if

• The sill or casing is soft, or the window frame itself has rotted.
• The cracking is on second-storey windows you would have to reach from a tall ladder.
• Alligatoring covers most of the house and the paint has to be stripped to bare wood, especially on a pre-1978 house.
• Water keeps getting in after you recaulk, which points to flashing around the window or a roof or gutter problem above it.

In Virginia, painting contracting is a licensed specialty that covers surface preparation, caulking, sanding, and cleaning preparatory to painting.^[6] To keep it from coming back, see how to keep exterior trim from rotting, and if you are unsure who to hire, read carpenter or painter for exterior trim.

Getting it fixed on Loclis

On Loclis you describe the trim job once, with close-up photos of the cracking, and local businesses quote it. Browse exterior painting, find painters around Blacksburg, or read more exterior answers.