A window sill that faces south or west takes more UV punishment than any other painted surface inside a home. Unlike walls that sit in diffuse light, a sill sits horizontal, directly under glass that concentrates solar heat throughout the day. Add the temperature cycling from cold glass in winter and condensation forming on the surface every morning, and you have a surface that standard interior latex simply was not designed to handle. Sills painted with flat or eggshell wall paint routinely chalk, blister, and peel within three to five years. The right paint and the right prep turn that failure timeline into a decade of clean, durable finish.

This guide covers how to sand and prep sills that take direct sun, which paint products actually hold up, how to brush casings without drips, and how to protect glass while you work.

Sanding and Prepping Window Sills That Get Direct Sunlight

The first question with any sill repaint is how much of the old paint is still bonded. UV and condensation cycles degrade adhesion from the surface down. A sill that looks intact may have a topcoat that passes a thumbnail scrape but fails under tape. Run the cross-hatch test: score a small grid with a utility knife, press two inches of painter’s tape firmly over the grid, and pull sharply at a 45-degree angle. If paint lifts off in the grid pattern, you need to get back to solid paint before recoating. Painting over a failing layer only delays the same failure by a season or two.

For sills with sound existing paint, 120-grit sandpaper is the right starting point. Sand with the grain on wood sills and flat across painted surfaces to create a mechanical profile that the new coat can grip. After 120-grit, step up to 180-grit to smooth out the scratches before painting. On painted MDF or pine sills where the old paint has minor scuffing or gloss, a 220-grit scuff is often enough to break the surface sheen and prepare for adhesion. Wipe the sanded surface with a clean, slightly damp cloth and let it dry fully before priming or painting.

Check the glazing compound where the glass meets the sill. On older double-hung windows, the glazing compound at the lower sash junction dries out, shrinks, and develops hairline cracks. Water infiltrates those cracks, migrates under the paint on the sill, and lifts it. Press the existing glazing compound gently with a fingertip. If it crumbles or moves, rake it out with a putty knife, clean the channel with 80-grit sandpaper, and apply fresh glazing compound. Let it skin over for at least two weeks before painting over it. The professional approach is to lap the final coat of paint 1/32 of an inch onto the glass at the glazing line, creating a continuous seal between the glass and wood that water cannot penetrate. That small overlap is correct technique, not a sloppy edge.

For bare wood or any area that has been sanded back to raw wood, prime before topcoating. A waterborne alkyd primer or a shellac-based spot primer on any resin-rich areas handles the transition from bare to painted surface without the paint absorbing unevenly.

Choosing a Hard-Wearing Paint for High-Touch Window Areas

Standard interior latex paint formulated for walls has gloss units in the 10-to-35 range and a film hardness suited to vertical surfaces with occasional cleaning. Window sills are horizontal surfaces that face direct UV, collect condensation from cold glass on winter mornings, and take direct hand contact every time someone opens or closes the window. That use pattern wears through standard latex within a few years.

Interior/exterior enamel is the professional recommendation for window sills precisely because it bridges both use environments. Two products lead this category:

Benjamin Moore Advance in satin or semi-gloss is a waterborne alkyd, meaning it carries the oil-like film hardness and leveling of traditional alkyd paint but cleans up with soap and water and meets VOC regulations across all US states. After a full 7-day cure at room temperature, the Advance film handles repeated condensation cycling without softening. The pencil hardness reaches HB to H after full cure, which is the hardness range of cabinet enamel. Full hardness takes up to 30 days, so avoid placing objects on the sill for at least a week after painting.

Sherwin-Williams Emerald Urethane Trim Enamel is the harder alternative. The urethane modification produces a film that resists scratching more aggressively than Advance and yellows less over time, which matters on white or off-white sills where any yellowing shows clearly. The recoat window is four hours, and the surface is ready for light contact within a day, making it a practical choice when a room cannot be taken out of service for multiple days. At approximately $109 per gallon, it costs more than Advance, but the durability justifies the premium on high-use sills.

Standard interior latex is not the right choice for sun-exposed sills even though it appears adequate when first applied. The UV exposure chalks the surface and degrades the binder starting in year two or three. What looked like a cost saving at the hardware store becomes a full repaint within half the expected time.

Semi-gloss sheen is the right choice for sills. The higher gloss units, in the 45-to-65 range, repel moisture better than satin, resist fingerprints, and make the surface easier to clean without scrubbing through the finish.

Brush Technique for Painting Window Casings Without Drips

Casings present a different challenge than sills. The sill is horizontal and relatively wide. Casings are narrow, vertical, and profiled, meaning they have edges, recesses, and changes in plane that catch paint and turn it into drips before it can self-level. The brush you choose and how you load it controls whether a casing coat looks sharp or looks like the finish on a budget rental unit.

The Purdy XL series in a 2-inch or 2.5-inch angled sash is the standard professional choice for window casing detail work. The angled trim on the bristles allows you to press the tip precisely into the edge where the casing meets the wall without loading the heel of the brush with paint that drips onto the wall surface below. The Wooster Silver Tip Angle Sash is an alternative with a thinner, more flexible filament that leaves fewer brush marks in enamel finishes. Both brushes hold a clean edge reliably when loaded correctly.

Loading technique is where drips originate. Dip the brush no more than one-third of the bristle length into the paint. Tap the loaded brush gently against the inside of the can to release excess paint rather than wiping across the can rim. Wiping loads paint into the heel of the brush where it builds up and releases as a drip when you apply pressure to the tip. The tap releases the excess without overloading the tip.

For panel casings with recessed detail, paint the recessed channels and inside edges first with the brush tip, then coat the flat face surfaces. This sequence prevents loading flat faces and then dragging that paint into recesses, which causes buildup in corners that eventually sags and drips. Work in short, controlled strokes along the length of each casing element rather than pushing against the grain of the profile. Pull each stroke out to a feathered end rather than stopping abruptly, which leaves a ridge that catches light.

Two thin coats beat one thick coat on casings. A thick first coat on a vertical profile drips. A thin first coat that dries in two to four hours creates a stable base that the second coat levels against without the sag risk.

How to Protect Glass Panes While Painting Window Trim

There are two approaches to protecting glass while painting window trim, and they suit different situations. Masking protects the glass before painting. The no-mask technique skips protection and cleans the glass after painting. Both approaches work; the right one depends on the volume of windows you are painting and your tolerance for cleanup afterward.

For masking, Associated Paint Masking Liquid H2O is the professional standard. It is a high-solids, low-VOC liquid applied by brush or foam roller directly to the glass surface in the area adjacent to the trim. Any overspray or brush contact with the frame that gets onto the liquid mask becomes part of the sealer coat rather than a problem to clean. Apply a minimum of two coats for sufficient film thickness, allowing four hours to overnight between coats. After painting the trim, score along the molding edge with a sharp utility knife and peel the film cleanly off the glass. The two-coat application is important because a thin single coat tears during removal rather than peeling in one clean piece.

The 3M Masking Film, which is a static-cling plastic sheeting, is a faster alternative for glass panes where you need to cover a large area quickly. It adheres to glass without adhesive, requires tape only at the edges, and comes off cleanly without the scoring step.

The no-mask technique is what experienced painters use on repetitive work: paint up to the glazing line, let everything dry, then run a fresh razor blade along the paint line to trim any overlap onto the glass. The important correction to the instinct to stay off the glass entirely is that the professional approach deliberately laps the final coat 1/32 of an inch onto the glass at the glazing compound junction. That micro-overlap creates a continuous painted seal between the wood and glass that prevents moisture from entering the joint. A thin blade angled at 45 degrees removes the visible glass smear cleanly in one pass without touching the paint on the molding itself.

For the sill face that terminates at the glass on double-hung windows, the same logic applies. Seal the joint with that small overlap of paint, then clean the glass. This detail is what prevents the gradual moisture infiltration that lifts sill paint from the inside out.

After painting, wait a minimum of 24 hours before opening the window for ventilation. Emerald Urethane and Benjamin Moore Advance both cure at room temperature and should not be exposed to weather or heavy moisture contact within the first 48 hours. For windows on the wet or rainy side of a house, waiting a full week before leaving the window cracked overnight protects the fresh film from premature moisture contact.