How to Paint Exterior Window Frames (Timber, uPVC, Aluminium)

Painting exterior window frames starts with one question: what is the frame made of? Timber, uPVC, aluminium and factory-clad frames each need completely different preparation, and the wrong primer on the wrong material is the single most common reason a window repaint peels within a season. Identify the material, prep for that material, then worry about brushes and cutting in.

Window frames are the fiddliest part of an exterior repaint. They are small, they have dozens of edges, they sit next to glass you do not want painted, they carry hardware, and they move. On top of that they are usually the most weather beaten surface on the house, because the sill catches every drop of rain that runs down the glass. This guide covers material identification, prep, glazing lines, hardware, sashes that paint shut, and the safety rules for anything above ground level.

If you are working through the whole exterior, window frames are one item in a longer list. See the full exterior trim painting guide for the sequence and the soffit and fascia guide for the work above the windows.

Step one: identify the frame material

Stand at the window and look closely. Run a fingernail along an edge. Tap it. The material tells you everything that follows.

Bare or painted timber

Timber frames have visible grain, joints at the corners, and often show old paint layers stepping back where previous coats have chipped. They take paint willingly, which is why almost every older house has painted timber windows. They also rot, and the rot always starts in the same places: the bottom rail, the sill, and the mitred joints at the corners where water sits.

Prep for timber means scraping off anything loose, sanding back the edges of sound paint so the transition is not a visible ridge, and priming every patch of bare wood before topcoat. Pay special attention to end grain. The cut ends of timber drink water like a straw, so the underside of a sill, the bottom of a stile and any exposed cut edge needs primer worked into it properly rather than a quick pass. Joints get the same treatment. If a corner joint has opened up, fill or caulk it after priming, not before, so the filler bonds to a sealed surface. Our caulking guide covers which gaps to seal and which to leave alone.

Soft timber is a repair job, not a paint job. Press a screwdriver tip into the sill. If it sinks in, the wood has gone and paint will only hide the problem until it fails again.

uPVC and vinyl frames

uPVC frames are smooth, have welded corners with a faint seam, and were never painted from the factory. They can be painted, but they need either a specialist adhesion primer made for plastics or a paint specifically rated for uPVC. Standard exterior wall paint has nothing to grip on a slick plastic surface, and it will peel off in sheets.

Before you buy anything, check the window warranty. Some uPVC window manufacturers state that painting or otherwise altering the frame surface affects the warranty on that unit. Do not assume, and do not assume the answer is the same for every brand. Find the paperwork for your specific windows or contact the manufacturer and ask directly. A repaint is not worth losing cover on a sealed unit.

If you do proceed, prep is a thorough degrease, a light abrade with fine abrasive to knock the sheen off, another clean to remove the dust, then the adhesion primer. Follow the primer data sheet on recoat times exactly. Adhesion primers usually have a window during which the topcoat bonds best, and going outside it costs you the bond you paid for.

Aluminium frames

Aluminium frames feel cold, sound different when tapped, and often carry a factory powder coat or anodised finish that has gone chalky. The problem with aluminium is not the metal, it is the factory finish, which is deliberately smooth and hard.

Prep is degrease first, because aluminium picks up an oily film, then abrade the factory finish so the primer has a mechanical key. Then use a self etching primer or an adhesion promoting primer rated for non ferrous metal. Skip the abrade step and you get a coating that looks perfect for a year and then lifts at every edge. If there is any corrosion, which on aluminium shows as white powdery pitting rather than orange rust, remove it back to sound metal before priming.

Factory finished and clad frames

Some windows are timber inside with an aluminium or composite cladding outside, and some are fully factory finished units with a baked coating. These are the ones where painting may genuinely be a bad idea. The factory finish outlasts anything you can apply by brush, so overcoating swaps a long life finish for a short life one and commits you to repainting forever. Check the warranty, and if the existing finish is sound, consider cleaning it rather than coating it. If it is failing, get a straight answer from the manufacturer about approved refinishing before you start sanding.

Lead paint on pre 1978 windows

If the house was built before 1978 in the United States, treat the existing paint on window frames and sills as lead until a test proves otherwise. Window frames are the classic lead surface. They were painted with durable oil based coatings for decades, they have many layers, and the friction of a moving sash grinds those layers into dust.

The paint itself sitting there is not the hazard. Sanding, scraping and power tools are the hazard, because that is what turns a stable film into breathable dust that settles in the soil under the window and on the sill where children put their hands. Test before you abrade anything. Our guide on how to test for lead paint walks through the swab and lab options. If the result is positive, stop and look up the certified renovator requirements that apply where you live before going any further. This is not an area for improvising.

Tools and materials

Brush choice matters more here than on any flat surface. A cheap brush with a blunt tip will not hold a line against glass no matter how steady your hand is. See how to choose a paint brush for what to look for in a sash brush. For the coating itself, our best paint for exterior trim guide covers the categories that hold up on frames, and do I need primer before painting explains when a primer is genuinely required rather than optional.

Ladder and fall safety for upper floor windows

Ground floor windows you can do standing up. Upper floor windows are ladder work, and falls are the biggest real injury risk in exterior painting. Take this section seriously.

• Set the ladder on firm, level ground. Soft soil under one stile is how ladders tip. Use a levelling foot or a solid board rather than a stack of bricks.
• Extend the ladder well past the working point so you have something to hold when you climb on and off.
• Keep your hips between the stiles. If you have to lean out to reach the far side of a window, get down and move the ladder. Every reach is one window frame. Every fall is a hospital visit.
• Use a ladder stand off or stabiliser so the ladder rests on the wall rather than on the window itself. A ladder leaning on glass or on a frame you are about to paint is a bad idea twice over.
• Hook the paint container to the ladder rather than holding it. Both hands should be available.
• Do not work on a ladder in wind, on wet rungs, or with wet paint on your soles.

Our guide to choosing a ladder for painting covers duty ratings and the right ladder for each height, and the painting safety equipment checklist covers eye protection and respiratory protection for the sanding stage.

Prep, step by step

1. Wash. Frames collect grime, cobwebs, pollen and mildew, especially under the top edge and in the sill corners. Wash and let dry fully. Paint over dirt fails at the dirt.
2. Remove failing paint. Scrape back to sound material. On timber, work with the grain. Do not chase a blister across the whole frame if the surrounding film is well bonded, but be honest about where sound paint stops.
3. Feather the edges. Sand the transition between bare substrate and remaining paint so it is not a ridge. Ridges telegraph through the topcoat and look worse than the original damage.
4. Repair. Fill small defects with an exterior grade filler. Cut out and treat rot properly. If the sill is soft through, replace it before painting. If you have thick failing layers everywhere, stripping the paint may be faster and better than chasing individual flakes.
5. Prime. Prime every bare patch with the primer that suits the substrate. Work primer into end grain, joints and the underside of the sill.
6. Caulk. Seal the joint between frame and wall, and any opened corner joints. Leave designed drainage gaps open. Weep holes at the bottom of a frame exist to let water out and must never be caulked shut.

Hardware: remove it or mask it

Take off what comes off easily. Handles, stays, catches and casement fittings usually come off with a screwdriver in under a minute each, and painting around them takes longer than removing them and looks worse. Bag the screws by window so you are not guessing at reassembly.

What stays on gets masked. Hinges are usually the awkward case, because removing a hinge means taking the sash out. Mask hinge knuckles carefully, and keep paint out of the pivot. Paint in a hinge stiffens the action and eventually cracks.

Cutting in against glass: the two schools

There are two legitimate approaches and painters argue about them endlessly.

Freehand with a guard

A good angled sash brush, loaded properly, held at a shallow angle, drawn steadily along the glazing line. A rigid paint guard held against the glass catches overrun. This is faster than taping once you have the hand for it, and on a house with a lot of small panes it is dramatically faster. The downside is that it takes practice, and a wobble shows.

Taping

Mask the glass with low tack tape right up to the glazing line, paint freely, then pull the tape while the paint is still slightly soft rather than fully cured. Taping is slower to set up but gives a predictable result, and on a large modern pane with one long straight run it is genuinely quick. On a Georgian style window with many small panes, taping every one is a long afternoon.

The sealing trick

Whichever method you use, let the paint lap a hair onto the glass, roughly the width of a pencil line, along the glazing line. That thin overlap seals the junction between putty or bead and the glass, which is exactly where water gets in and rots the frame from the inside. Once the paint has cured, run a fresh window scraper blade along the glass at a shallow angle and take the overlap off cleanly. You end up with a sharp line and a sealed glazing joint, which is the best of both.

Two rules on the scraping: wait until the paint is properly cured, not just touch dry, or you will drag it. And use a fresh blade. A dull blade skips and scratches glass.

Painting order: casement versus sash

Casement windows

Open the casement. Paint the parts that are only visible when the window is open first, working from the inside out: the rebate, the edges of the opening sash, the inner face of the frame. Then paint the outer face of the sash and the outer frame, and finish with the sill. Working inside to outside means you are not leaning across wet paint to reach the awkward bits.

Sash windows

Sliding sashes need the sashes moved partway through. Start with the upper sash pulled down so you can reach its lower rail and the meeting rail, paint what is exposed, then reposition the sashes to expose what you missed, paint that, and finish with the outer frame and sill. Leave the running surfaces, the sides of the sash that slide in the channel, unpainted or very lightly coated. Building up paint in the channel is exactly how a sash stops running.

Do not paint the window shut

This is the failure everyone remembers. A freshly painted window that has been closed on soft paint welds itself shut, and freeing it damages the finish you just applied.

• Do not paint the mating faces heavily. A thin coat on a rebate is fine. A thick one is glue.
• Break the seal repeatedly as the paint sets. Open and close the sash every so often through the drying period rather than leaving it shut and hoping.
• Time the job so the window can stay open long enough. See the weather section below.
• Wax or a dry lubricant on mating faces, applied before painting and kept off any surface you intend to coat, reduces sticking on sashes that have always been tight.

Breaking a painted shut sash

If a window is already welded shut from a previous repaint, do not just pull harder. Cut the paint film along the whole seam with a sharp blade or a dedicated window zipper tool, working right around the perimeter including the meeting rail. Then work a stiff putty knife into the seam and ease it along, tapping gently rather than levering hard in one spot. Levering in one place cracks glazing and splits timber. Once it moves, clean the built up paint out of the channels before you repaint, or it will stick again.

Weather and timing

Window frames have a constraint most exterior surfaces do not: at some point tonight the window has to close. That means you need enough dry, mild hours after you finish for the paint to set firm enough that closing it does not weld the sash.

Practically, that means starting early on a dry day with a decent forecast, doing the windows you can finish rather than opening every window in the house at once, and working the sashes as the paint sets. Avoid painting in direct hot sun, which skins the surface while the film underneath stays soft, and avoid late afternoon starts when dew will land on uncured paint. Temperature and humidity limits are on the product label, and the label is the authority. Our guide to the best weather conditions for exterior painting covers the general rules.

How many coats

Two topcoats over primer is the normal answer on frames. Frames are high exposure, high contact, and thin single coats on sharp edges wear through first. Apply thin coats rather than heavy ones. Heavy coats on a frame run into the glazing line, sag on the vertical stiles, and take much longer to harden, which brings the paint shut problem straight back. Respect the recoat time on the label between coats.

Common mistakes

• Priming plastic or metal with a wood primer because that is what was in the shed.
• Painting over live rot on the sill because it is easier than replacing it.
• Caulking the weep holes shut, trapping water inside the frame.
• Sanding pre 1978 frames dry with no testing and no containment.
• Leaning off the ladder to reach the far jamb instead of moving the ladder.
• Scraping the glass overlap off before the paint has cured, and dragging the line.
• Closing the window at dusk on soft paint.
• Skipping the underside of the sill because nobody sees it. That is where the water sits.

Frequently asked questions

Can I paint uPVC window frames?

Yes, with the right products, but check the window warranty first. uPVC needs a degrease, a light abrade and either a specialist adhesion primer for plastics or a coating specifically rated for uPVC. Ordinary exterior paint has nothing to grip and will peel. Some manufacturers treat surface alteration as affecting the warranty on that unit, so confirm with your specific supplier before you start.

Do I need to remove the old paint completely?

Not usually. Remove everything that is loose or flaking, then feather the edges of the sound paint so the transition is smooth. Full removal is only worth it when the build up is so thick that the sash no longer runs, when the film is failing across the whole frame, or when a previous coat is incompatible with what you are applying.

Should I tape the glass or cut in freehand?

Both work. Freehand with a good angled sash brush and a paint guard is faster once you have the technique, especially on windows with many small panes. Taping is more predictable and quicker on large single panes. Either way, let the paint lap a hair onto the glass to seal the glazing line, then scrape it off cleanly with a fresh blade once cured.

How do I stop the window painting shut?

Keep the coat on mating faces thin, work the sash open and closed repeatedly while the paint sets rather than leaving it shut, and plan the job so you have enough dry hours before the window has to close for the night. Never build paint up in the running channels of a sash window.

Do I have to worry about lead on old windows?

If the home was built before 1978 in the United States, assume the existing paint contains lead until testing says otherwise. Window frames and sills are classic lead surfaces. The danger comes from sanding and scraping, which turns the film into dust, so test before you abrade and follow the certified renovator rules that apply in your area if the result is positive.

How many coats do exterior window frames need?

Two topcoats over an appropriate primer is standard. Apply them thin rather than heavy. Thick coats sag on the vertical sections, run into the glazing line and stay soft for far longer, which makes the window much more likely to stick shut when you close it.