How to Sand Window Frames: Glass, Putty and the 2mm Paint Line

Window frames are the hardest sanding job in a house. Not because the technique is difficult, but because everything about them is awkward: narrow sections, glazing bars, glass you must not scratch, putty you must not break, and a bottom rail that is usually the most weathered timber on the building.

They are also the job with the highest payoff. Timber windows fail from the outside in, and they fail at the joints and the bottom rail. Well-maintained frames last a century. Neglected ones rot out in twenty years and cost a fortune to replace.

Two things catch people out. First, old window paint very often contains lead — windows were repainted more than almost any other surface in a house. Second, there is a deliberate paint line onto the glass that seals the putty, and sanding it off is how water gets in. ⚠️

Test for Lead Before Anything

This section comes first because on windows the risk is higher than almost anywhere else in a house.

Windows were repainted more often than any other joinery — every few years, for decades. That means old frames often carry many layers, and the oldest ones may well predate the 1978 restriction on lead paint.

Friction surfaces make it worse. Sash windows and casements rub as they open and close, which grinds lead-containing paint into dust at the very point where people put their hands and where air moves into the room. Window sills and frames are a well-recognised source of household lead dust for exactly this reason.

What to do:

• Test before sanding. Score through all the layers so the swab reaches the oldest paint, not just the top coat.
• If positive, do not sand. Sanding creates the fine dust that is most hazardous.
• Use chemical stripping instead, which removes paint as wet sludge — see removing paint from wood.
• Do not use a heat gun on lead paint either. High heat vaporises lead into fumes.
• Consider a certified professional, particularly if there are young children in the house.

The Parts of a Window

Knowing the names makes the rest of this easier.

• Frame — the fixed outer surround built into the wall.
• Sash or casement — the opening part that holds the glass.
• Stile — the vertical members of a sash.
• Rail — the horizontal members. The bottom rail is where water sits and where rot starts.
• Glazing bar (muntin) — the thin bars dividing a multi-pane window. The fiddliest part.
• Putty / glazing bead — what holds the glass in. Older windows use linseed oil putty; newer ones use timber beads.
• Sill / cill — the horizontal shelf at the bottom, sloped to shed water.
• Drip groove — a small channel under the sill that stops water tracking back to the wall. Do not fill it with paint.

Almost every window failure starts in the same three places: the bottom rail, the joints at the corners of the sash, and the seal between putty and glass. Spend your effort there.

Assess Before You Sand

Walk round each window and check before committing.

Probe for rot. Push a screwdriver gently into the bottom rail, the corners of the sash, and the ends of the sill. Sound timber resists. If it sinks in, you have rot — see the section below, because sanding will not fix it.

Check the putty. Is it sound, or cracked and falling out? Loose putty must be replaced, and that is a different job from painting.

Check the paint condition. Sound and just tired means keying. Flaking, blistering or bare patches means more work. Blistering on the outside often signals moisture getting in from behind.

Check the joints. Open corner joints on a sash let water straight in. They need filling or repair before finishing.

Check the glass. Cracked panes should be replaced before you paint, not after.

Check the weather. Exterior window work needs several dry days — time to strip, sand, prime and topcoat. Bare timber left exposed absorbs moisture fast.

Masking the Glass Properly

Sandpaper scratches glass permanently, and a sander will touch the glass. Masking is not optional.

1. Clean the glass first where the tape will sit. Tape does not stick to dusty glass and will lift mid-job.
2. Run the tape along the glass, right up against the putty or bead line, pressing the edge down firmly with a fingernail or a plastic card.
3. Leave the correct amount of glass exposed if you plan to paint onto it — see the paint line section below.
4. Use delicate-surface tape on old paintwork. Standard masking tape can pull flaking paint off when removed.
5. Do not leave tape on for days, especially in sunlight. Adhesive bakes on and becomes very difficult to remove.

An alternative to masking: a metal or plastic paint shield held against the glass as you work. Slower, but no tape to apply or remove, and it also protects during sanding. Many decorators use both — tape for painting, a shield for sanding.

Putty — Handle With Care

Traditional linseed oil putty holds glass into older frames. It hardens over decades but stays comparatively brittle, and it is easily damaged by sanding.

If the putty is sound: leave it alone. Sand around it and up to it, not across it. It only needs a light key at 180 so the paint sticks.

If the putty is cracked, loose or missing: it has to be replaced. Rake out the failed sections, prime the exposed rebate — bare timber sucks the oil out of fresh putty and makes it crumble early — then re-putty and let it skin over before painting. That skinning period is often a week or two.

If your window has timber beads instead: these are simpler. Sand them like any other trim, and check the seal between bead and glass, which is usually a bead of sealant rather than putty.

Which Tools for Which Part

The realistic combination is a detail sander for the bars and corners, sanding sponges for profiles, and a palm sander or random orbital for the wider flat sections and the sill.

The Right Grits for Windows

Exterior windows end at 180 before primer, same as any painted joinery. Our grit chart covers the reasoning, and sanding before painting explains why finer is not better here.

Step by Step, Start to Finish

Step 1 — Test for lead and check the forecast

Both before you start. Exterior work needs several dry days.

Step 2 — Clean the frames

Wash off dirt, algae and cobwebs. Sanding a dirty frame clogs abrasive instantly.

Step 3 — Remove or protect the furniture

Take off stays, catches and handles where you can. Mask what you cannot remove.

Step 4 — Mask the glass

Tape along each pane, pressed down firmly, leaving the paint line allowance if you are painting onto the glass.

Step 5 — Scrape off loose paint

A scraper removes flaking material faster than abrasive and saves your sandpaper. Work carefully around putty and glass.

Step 6 — Sand the flat sections

Palm sander or random orbital at 120 on failing areas, then 180 overall. Keep the pad flat and away from the glass.

Step 7 — Do the glazing bars by hand

Detail sander tip or folded paper along each bar. Work along their length. This is the slow part — accept it.

Step 8 — Feather the edges of any bare patches

Blend where old paint meets bare timber so no ridge shows through the finish.

Step 9 — Repair and fill

Fill cracks, open joints and small defects with an exterior-grade filler. Cure fully, then sand flush at 180.

Step 10 — Prime bare timber the same day

Exposed exterior wood absorbs moisture quickly. Prime it before it can, and prime end grain especially — the ends of a bottom rail are where water gets in.

Step 11 — Re-putty if needed, and let it skin

Then prime the putty before topcoating.

Step 12 — Two thin topcoats, with the paint line onto the glass

Light 320 pass between coats. Remove the tape while the final coat is still slightly soft for a crisp edge.

The Paint Line Onto the Glass

This is the detail almost nobody knows, and it matters more than any sanding technique in this guide.

On a traditional puttied window, the paint should overlap from the putty onto the glass by roughly 2mm. That is not sloppiness — it is deliberate and it is functional.

What that overlap does: it seals the joint between the putty and the glass. Without it, water runs down the pane and finds its way straight into the gap, soaks into the putty, and then into the timber rebate behind. That is one of the main routes by which timber windows rot from the inside out.

What this means for sanding: if you sand right up to the glass and remove that paint line without replacing it, you have opened the seal. Any restoration of a puttied window must re-establish the overlap.

How to get it neat:

• Mask the glass leaving about 2mm exposed beyond the putty line.
• Paint up to and slightly over the tape edge.
• Remove the tape while the paint is still slightly soft.
• Or paint freehand with a good angled sash brush and scrape any overshoot off the glass with a blade once dry — which is how professional decorators do it.

On modern bead-glazed windows this does not apply in the same way — the seal is a bead of flexible sealant rather than putty, and paint is kept off the glass. Check which type you have before deciding.

Rot, and When Sanding Is Not Enough

Sanding and painting protect sound timber. They do nothing for timber that has already failed.

How to find rot: press a screwdriver into the bottom rail, the corners of the sash, the ends of the sill and anywhere paint is blistering. Sound wood resists firmly. Soft, spongy or crumbly wood is rot.

Small areas: cut back to sound timber, treat with a wood hardener, then fill with a two-part exterior wood filler. Shape, sand at 180, prime immediately. Done properly this lasts for years.

Larger areas: a joiner can splice in new timber — cutting out the failed section and letting in a matching piece. This is standard, well-established repair work and far cheaper than replacing a window.

Widespread rot: be honest. If the bottom rail and both lower joints have gone, the sash may be beyond economic repair.

Common Mistakes (and How to Fix Them)

Mistake 1 — Not testing for lead

What happens: you generate lead dust at the exact spot where it collects on the sill.
The fix: test first, scoring through all the layers. If positive, strip chemically or hire a professional.

Mistake 2 — Sanding without masking the glass

What happens: permanent scratches on the pane.
The fix: tape or a paint shield, every time.

Mistake 3 — Removing the paint line onto the glass

What happens: the putty-to-glass seal opens and water gets in behind.
The fix: re-establish a 2mm overlap when repainting a puttied window.

Mistake 4 — Sanding over sound putty

What happens: it chips and crumbles, breaking a seal that was working fine.
The fix: key it lightly by hand and leave it alone otherwise.

Mistake 5 — Using a heat gun near glass

What happens: uneven heating cracks the pane.
The fix: chemical stripper near glass, or a heat shield attachment.

Mistake 6 — Painting over rot

What happens: decay continues invisibly and the repair fails within a season or two.
The fix: probe first, cut out soft timber, treat and fill properly.

Mistake 7 — Leaving bare timber exposed overnight

What happens: it absorbs moisture and dew, which then gets sealed under the primer.
The fix: prime bare wood the same day you expose it.

Mistake 8 — Filling the drip groove with paint

What happens: water tracks back along the underside of the sill to the wall.
The fix: keep the groove clear and clean it out if previous coats have filled it.

Pro Tips Worth Knowing

Prime end grain twice

The ends of rails and sills drink moisture. Two coats of primer there, thinned slightly on the first, is standard practice and prevents a lot of future rot.

Do the outside first, then the inside

The exterior is weather-dependent and the interior is not. Get the weather-critical work done while conditions allow.

Use a sash brush

An angled brush cuts a far crisper line against glass than a straight one, and reduces how much masking you need.

Work one window at a time, completely

Half-finished windows are vulnerable to weather. Finishing each one before starting the next means nothing is left exposed.

Keep the opening edges thin

Paint build-up on the closing edges makes a casement bind. Key those edges but avoid adding coats there.

Check the drip groove and repaint the sill last

The sill takes the most weather and benefits from an extra coat.

What I Recommend

The sanding tools for a window are ones you likely own. The item that decides whether the job looks neat or amateur is the tape.

Frequently Asked Questions

Your Final Checklist

• Lead tested if the property predates 1978, scoring through all layers
• Several dry days confirmed for exterior work
• Timber probed for rot at bottom rails, joints and sill ends
• Any soft timber cut out, treated and filled before finishing
• Putty condition assessed — sound, or raked out and replaced
• Frames washed and dried
• Furniture removed or masked
• Glass cleaned, then masked with delicate-surface tape
• 2mm paint-line allowance left on puttied windows
• Loose paint scraped before sanding
• Flat sections sanded at 120 then 180
• Glazing bars and corners done by hand or detail sander
• Bare patch edges feathered so no ridge shows
• No machine used near putty or glass
• Bare timber primed the same day, end grain twice
• New putty allowed to skin before painting
• Drip groove under the sill kept clear of paint
• Two thin topcoats with a light 320 pass between
• Paint line re-established onto the glass
• Tape removed while the final coat was still slightly soft

Related reading: how to use a detail sander, how to remove paint from wood, and how to sand a door.