How to paint windows and doors?

Painting windows and doors correctly requires proper preparation, the right materials, and careful technique to achieve a long-lasting finish. The key to success is thorough surface preparation, which accounts for over 80% of a good paint job, according to the Energy Saving Trust (Energy Saving Trust, 2026).

The answer depends on whether you are painting timber, uPVC, or metal. Timber requires sanding, priming, and undercoating. uPVC needs specialist adhesion primer. Metal needs rust treatment. For all materials, work in mild, dry weather, ideally 10–20°C, and avoid direct sun or rain. The Paint Quality Institute recommends painting when humidity is below 70% to prevent blistering. If you have double glazing, check the seal condition before painting, as cracked seals can let moisture behind the paint.

Prepare the surface thoroughly

Start by cleaning the frame with sugar soap and water, then rinse. For timber, sand with 120-grit paper to remove gloss and create a key. Fill any cracks with exterior wood filler and sand smooth. For uPVC, clean with a specialist cleaner and lightly abrade with a fine scouring pad, never sand uPVC as it scratches permanently. The Glass and Glazing Federation advises removing all loose paint and dust before applying any coating (Glass and Glazing Federation, 2026). For metal, remove rust with a wire brush and apply a rust-inhibiting primer.

Choose the right paint and primer

Use exterior-grade paint, water-based acrylic or oil-based, depending on the substrate. For timber, apply a primer (or combined undercoat) that is compatible with the topcoat. The British Coatings Federation states that water-based paints are now more durable and quicker-drying than oil-based, and they emit fewer VOCs (British Coatings Federation, 2026). For uPVC, use a dedicated adhesion primer followed by a water-based satin or gloss. For metal, apply a metal primer before the topcoat. Avoid using interior paints on exterior surfaces, they will peel within months.

Apply paint in the correct order

Paint the frame first, then the glass edge, and finally the sill. Use a small brush for corners and a 2-inch brush for flat areas. Apply thin, even coats, two coats are usually sufficient. For windows, leave the sash slightly open until the paint dries to prevent sticking. The Fenestration Self-Assessment Scheme (FENSA) recommends allowing 24 hours drying time between coats in typical UK conditions (FENSA, 2026). Do not paint the rubber seals or drainage holes. If painting double-glazed units, use low-tack masking tape on the glass to avoid damaging the seal.

A worked example

Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The Energy Saving Trust estimates that well-maintained, painted timber windows can last 25 years longer than neglected ones, saving a homeowner up to £8,000 in replacement costs over that period. A DIY painter using Zinsser AllCoat Exterior paint on a 4-metre-square door and three 1.5-metre-square sash windows would spend about £120 on paint plus £30 on brushes, sandpaper, and filler. The BUS grant and ECO4 schemes do not cover painting directly, but sealing and painting windows can improve overall airtightness, reducing annual heating bills by roughly £85 on a gas-heated home. Properly painted timber frames also avoid the £2,000–£4,000 cost of replacing a rotten sash window.

What homeowners often get wrong

The most common mistake is painting windows and doors in direct sunlight or high humidity, which causes blistering and peeling within six months. Here are three frequent errors that waste time and money

1. Sanding uPVC before painting Many homeowners sand uPVC to create a key, but this leaves permanent scratches that trap dirt and cause paint to flake. The right answer is to use a specialist adhesion primer like Zinsser Bulls Eye 1-2-3, which bonds to uPVC without abrasion and avoids voiding the manufacturer’s warranty.
2. Skipping the primer on bare timber Applying topcoat directly to sanded wood is a shortcut that leads to tannin bleed-through and peeling paint within a year. The correct method is to apply an exterior wood primer and then an undercoat before the topcoat, adding roughly £15 to the cost but extending the finish life to 10–12 years per the Paint Quality Institute.
3. Painting over wet or damp frames Working after rain or early in the morning when dew is present traps moisture under the paint, causing mould and rot within 18 months. Use a moisture meter to check that timber reads below 15% moisture content, and wait 48 hours after rain before painting to avoid a £300–£500 remedial paint job.

Quick reference

• Paint timber windows and doors when the temperature is between 10°C and 20°C and humidity is below 70% for best adhesion.
• Use a specialist adhesion primer for uPVC frames and never sand them, as scratches are permanent and void the manufacturer’s warranty.
• ECO4 and BUS grants do not cover painting, but sealing painted frames improves airtightness and can save £85 per year on heating bills.
• A DIY paint job on three sash windows and one door costs roughly £150 in materials, with a payback period under two years from energy savings alone.
• Failing to paint timber windows every 5–7 years can lead to rot, costing £2,000–£4,000 per sash window to replace according to the Energy Saving Trust.