The glass in a wood window frame needs protection from paint, but it also needs to surrender a thin strip of coverage where it meets the wood. That 1/32-inch overlap of paint onto the glass is not sloppiness. It is intentional. Sealing the joint between the wood sash and the glazing compound, and between the wood and the glass itself, prevents moisture from entering the frame at the most vulnerable spot: the junction where two different materials expand and contract at different rates. Painters who cut perfectly to the glass edge and leave no overlap onto the glazing are leaving the wood-glass junction open to water intrusion.

This distinction between windows and most other masking jobs defines the approach to exterior window painting. The goal is not to keep paint entirely off the glass. The goal is controlled paint placement that protects wood completely while keeping the glass clean enough to function without visible obstruction.

Masking Glass and Surrounding Surfaces Before Painting

Two methods exist for masking glass on exterior windows before painting: physical tape masking and liquid masking film. Each has specific applications where it works best.

Physical tape masking with ScotchBlue tape is the standard approach for hand painting with a brush when a painter is working carefully and cutting to the line. ScotchBlue Ultra Sharp Lines tape tears at 90-degree angles cleanly, which makes it well-suited for the square corners of window panes. Apply the tape along the glass edge with the tape line set back 1/32 to 1/16 inch from the wood, so the paint can lap slightly onto the glass and seal the joint. Press the tape edge firmly against the glass to prevent paint from bleeding underneath.

The ScotchBlue Tape Applicator tool speeds application along straight trim runs. The dispenser holds the tape roll and applies even tension as the tape is pressed against the surface, which produces a straighter line than hand-unrolling and pressing.

For large spray masking operations, a 3M masking machine paired with 3M 2080 Blue Edge Lock tape and masking film covers entire window units efficiently. The machine applies tape and film simultaneously in one pass, which is substantially faster than taping and then separately covering with plastic sheeting.

Liquid masking film is the professional sprayer’s tool for large window glass areas. Associated Paint Masking Liquid H2O is a high-solids, low-VOC product that applies directly to glass with a brush, roller, or sprayer. It dries to a peelable film over the glass, protecting it from overspray. After painting, scribe the edge of the film with a utility knife along the sash and peel the film off. The dried film lifts cleanly without leaving residue on glass.

One critical limitation of liquid masking on exterior windows: do not use it on exterior wood windows in a way that covers the wood frame. Proper exterior window technique requires painting the wood lap onto the glass, not masking the wood-glass junction entirely. Liquid masking can cover the center of glass panes while leaving the perimeter junction unmasked for the intentional glass overlap.

Do not leave liquid masking film on exterior glass for more than 30 days, particularly in hot or dry climates where the film may harden and become difficult to peel.

Brush Technique for Exterior Window Frames and Muntins

Brushing exterior window frames requires a quality sash brush and a specific loading and cutting technique. The sash brush, an angled brush in the 1.5 to 2.5-inch range, is designed for exactly this work. Its angled tip allows precise cutting along the glass line and into the narrow channels around muntins.

For cutting against glass, load the brush to about one third of the bristle length. Overloading causes runs that flow down the glass face and pool at the bottom of the frame. Touch the tip of the brush to the frame surface two to three inches from the glass line and draw the brush toward the glass edge. As the bristle tip approaches the glass, ease pressure and allow just the tip to make the cut. A well-loaded brush with light tip pressure deposits paint without pushing excess onto the glass.

Muntins, the dividing bars between individual panes in multi-lite windows, require the narrowest brush and careful attention to avoid bridging paint across the full width of the bar. Apply paint to the face of the muntin first, then the returns on either side. Avoid overfilling the joint between the muntin and the glass glazing compound. The glazing compound should be sealed with paint, but paint pooled in excess at the joint is unsightly and can trap moisture in the recess.

Work from top to bottom on window frames. Drips and runs from upper sections land on areas that have not yet been painted, which means they can be incorporated into the wet coat rather than dried onto finished sections.

Painting Shutters On or Off the Wall

Painting shutters while they are still mounted on the house is always a compromise. Mounted shutters present their back face, side edges, and the siding immediately around them as masking challenges. The result is typically incomplete coverage, especially on the back face and edges, which are the first surfaces to peel.

Removing shutters and hanging them off the house is the superior method. S-hooks on a length of steel wire strung between two trees or posts allow the shutters to hang freely with all surfaces accessible. Using an airless sprayer on free-hanging shutters provides coverage on all faces, including the back and all edges, in a fraction of the brush time needed for mounted shutters. Rotate the shutters while spraying to cover the full perimeter. Allow them to dry hanging before stacking.

Painting shutters lying flat on sawhorses or on the ground is an alternative to hanging. Spray or brush one face and all edges on the first pass. Allow to dry and flip. The second pass covers the back face and cleans up any missed edge coverage. The disadvantage of lying flat versus hanging is that runs on vertical surfaces are not a concern when horizontal, but the back face needs a separate setup.

If shutters must be painted while mounted, mask the surrounding siding with painter’s tape and plastic sheeting. Remove any hardware that can be removed. Use a brush or mini-roller rather than a sprayer on mounted shutters unless the surrounding siding is also being spray-painted, in which case the shutters are simply included in the spray pass.

Preventing Overspray When Using an Exterior Sprayer Near Windows

Airless sprayer overspray drifts significantly, especially on windy days, and glass is one of the least forgiving surfaces to clean overspray from because dried paint bonds to glass and requires razor blade removal.

The first line of defense is wind management. Do not spray near windows when wind is above 10 miles per hour. Overspray at moderate wind speeds drifts substantially farther than expected and can deposit a fine mist of paint on windows 20 feet away from the spray area. The mist is thin enough to be nearly invisible when wet but becomes clearly visible when dry.

Adjust the tip size to match the application. For trim and frames, a 411 or 413 tip produces a narrower fan pattern with better control than the 515 tip used for large siding areas. A narrower fan reduces the amount of overspray generated per square foot of coverage.

Reduce pressure to the minimum needed to atomize the paint correctly. Excessive pressure produces finer atomized droplets that stay suspended longer and drift farther. At correct minimum pressure, the spray pattern is well-formed without the feathering at the edges that indicates over-atomization.

Hold the spray tip at the correct working distance from the surface, typically 10 to 12 inches for a 411 or 413 tip. Moving the gun closer increases deposition on the target area and reduces drift. Moving it farther increases the proportion of paint that becomes airborne overspray rather than landing on the surface.

For windows near the spray area that cannot be easily masked, the no-mask pro technique handles cleanup efficiently. Paint the frames completely, allowing overspray to land on the glass. After the paint dries, use a fresh single-edge razor blade in a holder to scrape the glass clean. A razor blade on dry latex paint on glass removes it quickly and completely without scratching the glass when the blade is sharp and held at a low angle to the glass surface. Change blades frequently. Dull blades scratch rather than cut.

The razor technique is faster than masking for experienced painters working with a brush. For sprayers, it works on light overspray but should not substitute for proper masking when heavy overspray is expected.