Learn how to find the moisture source behind swollen, peeling window trim, decide whether to repair or replace it, and finish with the right caulk, primer and paint. Includes lead-safety steps for older homes.
Fix the moisture source first, then cut out soft wood, prime bare spots, caulk the gaps with a flexible paintable sealant and repaint with a moisture-resistant acrylic enamel. Interior window trim that is only cracked or peeling can usually be repaired. Trim that is soft, crumbling or rotten across more than about 10 percent of a piece should come out. Test old paint for lead before you sand anything.
Why does interior window trim swell, crack and peel in humid weather?
Water gets behind the paint film and pushes it off from the back. According to Sherwin-Williams, high moisture exposure on a wall, ceiling or trim lets moisture penetrate the film until it pushes the paint away from the surface.
At a window there are three usual sources. The first is condensation. The BC Housing condensation guide explains that moist air cools against cold surfaces, and that condensation forms first on the coldest ones, usually window and door glass. Water that runs down the glass lands on the sill and the lower casing.
The second is failed caulk. Poorly sealed framing, or caulk that has cracked or peeled away, lets water undermine the coating. The third is a humid room with no ventilation. Showers and kitchens without exhaust fans keep trim damp for hours at a time.
Is it moisture damage or just a paint failure?
Peeling that starts at the glass or the caulk line, with bubbles, dark wood or soft spots, points to moisture. Peeling over sound, dry wood points to prep or paint.
Dunn-Edwards' paint problems guide lists the causes of blistering. They include painting in direct sunlight or on a hot surface, oil-based paint over a damp surface, and moisture passing through walls from bathrooms, kitchens and laundry rooms. It advises breaking open a blister and examining the substrate and the back of the paint. That small test tells you a lot.
| What you see | What it likely means | What to do |
|---|---|---|
| Peeling or bubbles at the glass line or caulk joint | Water is getting behind the film | Fix the condensation or caulk first |
| Blisters with damp or dark wood underneath | Moisture, not a paint problem | Dry the wood and find the source |
| Blisters with dry, bare wood underneath | Heat, sun or a damp surface at painting time | Scrape, prime and repaint under better conditions |
| Peeling over dry, sound wood | Prep or paint adhesion | Scrape, sand, clean and prime |
Can swollen or soft trim be dried and repaired?
Yes, if the wood is sound and dry. Probe it with a screwdriver tip. Wood that resists is a candidate for repair, while wood that sinks in or crumbles is not.
For a repair, cut out the rot-softened wood and let the area dry completely. This Old House quotes a restoration carpenter who checks exposed wood with a moisture meter and wants less than 18 percent moisture content before mixing epoxy. Too much moisture prevents a good bond and can lead to ongoing rot beneath the repair.
Then rebuild the void. Bondo Wood Filler is a two-part, non-shrinking, paintable filler. Its maker pairs it with Bondo Rotted Wood Restorer to fully repair rotted or soft wood. Sand the cured repair smooth before priming.
When should trim be replaced instead?
Replace it when it is crumbling, or when more than about 10 percent is rotten. That threshold comes from a sill repair at This Old House, where a simple epoxy repair suits small areas of rot but a sill with more than 10 percent rot should be cut off and replaced. The same logic is a fair guide for casing and aprons.
Use this pass/fail check before you commit to a repair:
- The probe stops at firm wood, so it passes. A probe that sinks in, or wood that crumbles, means replace.
- Rot covers less than about 10 percent of the piece, so it passes. More than that means replace.
- A meter reads under 18 percent in the exposed wood, so it passes. Wood that stays wetter after drying time means find the leak, then reassess.
If you replace a piece, prime the cut ends before installing. Sherwin-Williams says this reduces moisture penetration. It also advises against painting over cracked trim, because the cracking may persist and give the finish an unattractive texture.
How do you prep trim for repainting?
Fix the moisture source, scrape loose paint, test adhesion nearby, sand gloss, clean and prime bare spots. Ventilation fixes most interior moisture problems, and Sherwin-Williams recommends exhaust fans in bathrooms with showers. It also notes that dehumidifiers may help prevent peeling from coming back.
Remove as much failing paint as you can by scraping or sanding. Then test the coating in a six to 12 inch radius around the peeled areas to be sure adhesion is adequate. If paint lifts there, keep going until you reach paint that holds. Sand the edges of the remaining paint to feather them, and sand any glossy trim so new primer can grip. Wipe the dust off with a damp sponge and let it dry. Prime every bare patch, and be extra thorough around angles and crevices.
How do you clean mildew off trim before painting?
Clean it off and dry the surface, then fix the humidity feeding it. Paint over mildew or damp wood will fail again. Wash dark spots with a household mildew cleaner or a diluted bleach solution, rinse with clean water and let the trim dry fully, which can take longer in a South Florida summer. Wear gloves and eye protection, keep the room ventilated and never mix bleach with other cleaners. If the stain returns within weeks, look for a leak or a window that sweats every morning.
Should old trim paint be tested for lead before sanding?
Yes, for older homes. The U.S. Department of Energy's wood window guideline says lead is commonly found on old wood windows and trim, and that work on window systems should follow all state and federal laws for hazardous materials.
Use a test kit or a certified lab before you disturb the paint. If lead is present or you cannot tell, hire a lead-safe certified contractor, since many jurisdictions regulate paid work on older homes. Common safe-work practice also applies:
- Seal off the room with plastic sheeting and keep children and pets out.
- Mist surfaces and scrape wet. Never dry-sand or use a heat gun.
- Clean up with a HEPA vacuum and damp wipes, not a broom.
Which caulk works best on interior window trim in humid conditions?
Use a waterproof, paintable sealant rated for kitchens, baths or windows. Sherwin-Williams points to a waterproof latex kitchen and bath sealant for closing openings after peeling paint comes off. Check that the label says paintable and mildew resistant.
Caulk goes on after repairs and priming, on clean, dry joints. Cut the nozzle tip small, run one steady bead and tool it flat with a damp finger. Keep the bead thin, since a thick bead shrinks and splits. Fill the gap between trim and wall and the joint at the glass side where water collects. Let the caulk cure fully before painting. In humid weather that takes longer than the label's best-case time, and painting early can wrinkle the bead.
Which primer and paint resist moisture and mildew on trim?
Prime bare wood, then use a moisture-resistant acrylic enamel in semi-gloss. For window sills, Sherwin-Williams suggests an oil-based interior primer and a waterborne acrylic enamel. For moist rooms it suggests Duration Home interior acrylic latex, which it says is formulated with Moisture Resistant Technology.
Summer humidity slows cure time, so allow extra dry time between coats instead of rushing a second coat onto a damp surface.
We specify moisture-resistant acrylic paints with a washable finish in kitchens, baths and laundry rooms, and semi-gloss on trim and doors. If you manage an office, our Parkland commercial painting page covers that service area, and our guide to what a commercial interior quote covers explains the line items. For homes, our interior painting cost guide shows what drives the price of trim work, and if you want a crew to look at your window trim, start with our cost guide for cabinet and trim work and request an estimate.