6 minute read -
Sanding Window Frames: The Best Sander and Method
A detail sander is the best choice for sanding window frames, because its triangular pad reaches into corners and rebates. For the wider parts of the frame you will work faster with a sheet sander or random orbit sander, and a multi-sander combines both in one tool.
If the old paint is sound, key it with 120 to 180 grit. If it is flaking, remove the loose paint first and then sand from 80 to 120 grit.
What will you learn in this article?
- Which sander suits a window frame?
- Why use a cordless sander?
- Which grit should you use?
- Do you need to strip the frame to bare wood?
- How to sand window frames step by step
- When should you sand and paint exterior window frames?
- Safety
- Common mistakes
- Frequently asked questions about sanding window frames
- In short: sanding window frames
Which sander suits a window frame?
| Machine | Where on the frame | Why |
|---|---|---|
| Detail sander | Corners, rebates, along glazing beads | The point gets in everywhere |
| Sheet sander | Jambs, sills, head | Flat pad for an even finish |
| Random orbit sander | Wide sections, stripping to bare wood | Removes paint quickly with a fine finish |
| Multi-sander | Everything, with interchangeable pads | One machine for flat areas and corners |
A belt sander is usually too aggressive and too big for window frames. It is easy to take off too much, especially on edges and mouldings.
Why use a cordless sander?
Window frames are often a long way from a socket: on the outside wall, up a ladder or on scaffolding. With a cordless sander there is no extension lead to snag on the ladder. A sander from the Batavia Maxxpack 18V range runs on the same battery as your drill or saw. Take a second battery if you are doing a whole elevation, because sanding draws power continuously.
Which grit should you use?
- 60 to 80 grit: removing old, thick or flaking paint.
- 100 to 120 grit: smoothing bare wood and feathering the edges of old paint.
- 150 to 180 grit: keying sound paint before a new coat.
- 240 grit: light sanding between primer and top coat.
Do you need to strip the frame to bare wood?
Not if the paint is sound. In that case you just sand the old coat to a matt finish so the new paint can grip. Stripping back to bare wood is necessary if large areas of paint are lifting, if it is cracked, or if there are so many layers that the mouldings have filled in.
There is a middle way: remove only the loose paint and feather the edges of the remaining paint flat, so they no longer show through the new coat.
How to sand window frames step by step
Clean
Wash the frame with water and a degreaser or sugar soap. Outside there is often green algae and grime from traffic fumes. Rinse and let the wood dry thoroughly.
Check for wet rot
Push a bradawl or screwdriver into the wood, especially at the bottom of the jambs and in the sill. If the point sinks in easily, the wood is rotten. Cut the rot out back to sound timber, treat it with a wood hardener and fill with a two-part wood filler. Sand the filler flush once it has cured.
Remove loose paint
Scrape off loose paint with a paint scraper or a triangular scraper. For larger areas you can use a heat gun. Keep it well away from the glass, because the heat can crack the pane.
Sand
Start on the flat sections with the sheet sander or random orbit sander. Then do the corners and rebates with the detail sander. On bare wood, sand with the grain. Keep the sander flat and do not tip it over the edge, or you will round off the crisp corners.
Remove the dust and paint
Brush off the dust and wipe the frame with a tack cloth. Prime bare wood and filler, then apply two top coats. Mask the glass with painter's tape or let a thin line of paint run onto the glass, so water cannot get between the sealant and the wood.
When should you sand and paint exterior window frames?
Choose a dry spell with temperatures above about 10°C, without full sun on the frame. The wood must be dry. Do not paint early in the morning while there is still dew, or late in the afternoon when condensation can form. Late spring and late summer are usually the best times.
Safety
Wear a dust mask with a P2 filter, safety glasses and work gloves. Set the ladder on firm, level ground and do not reach further than your arm comfortably allows.
Old sealant around window frames can contain asbestos. The UK banned asbestos in 1999, so buildings built or refurbished before 2000 may contain it. Do not cut or sand old sealant yourself until it has been tested.
Common mistakes
- Sanding sound paint with 60 grit, which leaves visible scratches.
- Painting over wet rot instead of repairing it first.
- Pressing too hard in the corners, so the detail pad damages the moulding.
- Painting onto damp wood.
- Leaving dust on the frame before painting.
Frequently asked questions about sanding window frames
What is the best sander for window frames?
A detail sander for corners and rebates, ideally alongside a sheet sander or random orbit sander for the wider sections. A multi-sander with interchangeable pads is a good compromise if you want a single machine.
Can I sand window frames by hand?
Yes, for a few frames you can use a sanding block and sanding sponge. It does take a lot more time and effort. For a whole house, a machine will save you hours.
Which grit should I use to key window frames?
Use 150 to 180 grit if the old paint is sound. If loose paint needs to come off, start with 80 and work up through 120 to 150 or 180.
Do I need to sand exterior window frames every year?
No. But do check them every year for small cracks, flaking paint and wet rot. A well-maintained frame typically needs repainting every few years, depending on how exposed it is to sun and rain.
In short: sanding window frames
Use a detail sander for the corners and a flat sander for the wider parts, or a multi-sander that does both. Key sound paint with 150 to 180 grit and only strip back where the paint is failing. Repair wet rot before you paint, and never dry-sand old paint that may contain lead.
