Refinishing or replacing a windowsill during a renovation

How to tell whether a windowsill can be repaired

Start by working out what the sill is made of. Older Estonian flats and wooden houses in areas such as Kalamaja or Nõmme often still have the original painted wood sill, sometimes decades old, while newer developments and replacement windows almost always come with PVC. Tap it and look at the edges: wood has grain and can be planed or filled, PVC is a hollow-ish plastic shell with a smooth, slightly flexible surface.

A wood sill is worth refinishing if the timber underneath is sound: no soft, spongy patches, no rot where it meets the window frame, and only surface damage such as chipped paint, scratches or a stain from a plant pot. Press a screwdriver gently into any dark or damp-looking area; if it sinks in easily, the wood has started to rot and sanding will not fix it.

A PVC sill is worth refinishing only if the damage is cosmetic, and worth replacing if it is cracked, sagging, or pulling away from the wall. Cracks in PVC do not get better with paint, they usually widen with temperature changes, so a cracked sill is a replacement decision rather than a finishing one.

What you need to sand and repaint a wood windowsill

For a sound wood sill the job is straightforward and fits well into a DIY weekend. You need 120-grit and 220-grit sandpaper, a sanding block, wood filler for any chips or old screw holes, a scraper for loose paint, a tack cloth or damp rag, primer suited to wood, and a water-based or oil-based enamel finish depending on what is already on there.

Strip any flaking paint with the scraper first, then sand the whole surface with 120-grit to key it and remove the worst of the old finish, working along the grain. Fill any gouges or holes, let the filler cure, then sand it flush and go over the whole sill with 220-grit for a smooth surface before wiping away the dust.

Apply primer and let it dry fully before the top coat; two thin coats of enamel give a harder, more even finish than one thick one. Keep the window closed while it dries and avoid touching the sill for a full day or two, since paint that feels dry to the touch is often still soft underneath.

• 120-grit and 220-grit sandpaper plus a sanding block
• Scraper for loose or flaking paint
• Wood filler and a filling knife
• Primer suited to wood
• Water-based or oil-based enamel top coat
• Tack cloth or lint-free damp rag

Why painting a PVC sill usually is not worth it

PVC is not designed to take paint well. Its surface is smooth and slightly flexible, so most standard paints do not bond properly and start peeling, chipping or bubbling within months, often faster on a south-facing window that gets direct sun and heat. There are specialist plastic primers that improve adhesion, but the result still tends to be less durable than the factory finish.

The other issue is colour matching and finish: a painted PVC sill rarely looks as clean as an unpainted one, and once the paint starts to lift at the edges it looks worse than the stain or scuff you were trying to cover. For a sill that is only dirty or lightly scuffed, a thorough clean with a plastic-safe cleaner often does more good than paint.

If a PVC sill is genuinely damaged, cracked or discoloured beyond cleaning, replacing it is usually more sensible than trying to refinish it. New PVC sills are cut to size and are not an expensive part of a window job.

Where the windowsill fits in the renovation sequence

The sill sits between wall work and floor work, so its place in the sequence matters. Replaster or skim the wall around the window reveal first, since that work is messy and any dust or splashes on a finished sill mean redoing it. Fit or refinish the sill once the plaster is dry and before painting the walls, so any final touch-up paint on the sill edge matches the new wall colour rather than the old one.

If you are replacing the sill rather than refinishing it, do that before the walls are painted too, since fitting a new sill can chip plaster or leave gaps that need a bead of sealant and a touch-up. Flooring, in most cases, comes last, after the sill and walls are done, particularly if the new floor will sit flush against the wall below the window.

If the room is getting a full repaint as part of the same job, our painting and decorating service, priced from 12 € per job, can take care of the sill finish alongside the walls so the timing and the colour match up without you having to coordinate two separate jobs.

The most common mistake made during this kind of repair

The mistake we see most often is painting straight over old paint without sanding it back or dealing with what is underneath. New paint applied over a chipped or flaking surface just telegraphs the same chips and unevenness through the top coat, and it tends to fail again at the same spots within a year.

The second version of the same mistake is skipping primer to save time. Enamel top coats applied directly to bare or filled wood without primer soak in unevenly, leaving patchy sheen and poor coverage, which usually means a second full coat to fix what a primer coat would have prevented in the first place.

Both mistakes come from treating the sill as a five-minute touch-up rather than a small job with its own proper steps. Given how visible a windowsill is at eye level next to a window, it is one of the worst places in a room to cut corners on preparation.

When to stop and call in a specialist

If a wood sill has soft or rotten sections, particularly where it meets the window frame or an exterior wall, sanding and filling will not solve the underlying problem, and the rot can spread further into the frame. At that point it makes more sense to have the sill or the window looked at properly rather than repeatedly patching the same spot.

If the sill is loose, sloping, or pulling away from the wall, or if a PVC sill is cracked at the point where it meets the window frame, this usually points to a window fitting or sealing issue rather than a finishing one. Our window fitting and servicing, from 90 € per job, covers replacing a sill along with adjusting or resealing the window itself where the two are connected.

As a general rule: cosmetic damage on a sound sill is a weekend DIY job, but rot, structural movement or cracking at the frame is worth a proper look before it is covered up with paint or filler.