How to Prepare Woodwork for Painting: Sanding, Priming & Undercoating

Summary

Woodwork preparation is where most painting failures start. A decorator who rushes the prep on doors, skirtings, and window frames is setting themselves up for callbacks within 12-18 months. The paint film on woodwork is under constant stress from timber movement, temperature change, and mechanical wear — which means adhesion to the substrate is critical.

The two most common problems on woodwork are bleeding knots and peeling paint on previously painted surfaces. Knots contain resin that expands and contracts with temperature changes, eventually breaking through any paint system not properly sealed with knotting solution. Peeling on repaints nearly always traces back to insufficient surface preparation — inadequate cleaning, no deglossing, or paint applied over contaminated surface.

This guide covers both new/stripped bare wood and repainting previously painted joinery, including doors, skirting boards, window frames, and architraves.

Key Facts

• Knotting solution — shellac-based; seals resin in knots before priming; apply 2 coats; essential on softwood
• Wood primer — penetrating primer that soaks into bare timber; provides adhesion base for subsequent coats
• Aluminium wood primer — heavy-duty primer for resinous woods (pine, larch) and for filling grain on hardwood
• Water-based wood primer — quick-drying alternative; less effective grain penetration than oil-based; suitable for MDF and prepared hardwood
• Undercoat — opaque, high-build; builds film thickness and provides uniform base for topcoat; must be used before gloss
• Gloss finish — oil-based traditional gloss; longer dry time (16 hours recoat) but harder, more durable film than water-based
• Water-based satinwood / gloss — faster recoat (2-4 hours); lower VOC; slightly less durable; fine for most interior joinery
• MDF edges — highly porous; must be sealed with wood primer or specialist MDF primer before painting; may need 2 primer coats on edges
• Deglossing — previous glossy surface must be abraded (120-150 grit) or treated with liquid deglosser before repainting; paint will not bond to gloss
• Sugar soap — cleans grease and contamination before sanding; always clean before deglossing
• Grain filling — oil-based or water-based grain filler smooths open-grained hardwood; applied after primer, before undercoat
• Wet and dry sandpaper — used with water to get ultra-fine finish between topcoats; 400-600 grit

Quick Reference Table

Detailed Guidance

Sanding Sequence for New Wood

Always sand with the grain. Sanding across the grain creates scratches that show through the final finish.

1. 80-100 grit — only for rough-sawn timber or removing mill marks; not needed on prepared joinery
2. 120 grit — first sand on dressed timber; removes arris (sharp edges), smooths surface
3. 180 grit — follow-up sand; raises any grain and creates a uniform surface for priming
4. After primer: 240 grit — light sand to remove grain raised by primer
5. After undercoat: 320 grit — smooths undercoat texture before topcoat
6. Between topcoats (optional): 400 grit wet-and-dry — for a fine furniture-quality finish

Remove all dust with a tack cloth before each coat.

Knotting Solution — When and How

Apply knotting solution (shellac in methylated spirits) to every visible knot in softwood. Apply two thin coats with a small brush, allowing 15 minutes between coats. Do not prime over wet knotting solution — allow at least 30 minutes.

If a knot bleeds through a finished paint system, sand back, apply knotting solution over the bleed, allow to cure, then prime and repaint. Commercial knotting solution brands include: Rustins Knotting Solution, Zinsser Bulls Eye Shellac, and proprietary products from major trade suppliers.

Filling and Stopping

• Interior woodwork — use a fine surface filler or wood stopper for small holes and dents; apply after priming, sand flush when dry
• Exterior woodwork — use an exterior flexible wood filler (e.g. Bostik Wood Filler Exterior) that flexes with the timber; standard fillers crack on exterior joinery
• Gaps at skirting boards — fill with a paintable flexible acrylic sealant (e.g. No More Gaps), not rigid filler; apply after painting for cleanest finish, or apply before and smooth to a fine line

Do not overfill — multiple thin coats dry better than one fat application. Sand flush with 180 grit when fully cured (follow manufacturer's dry time — some fillers need 2-4 hours before sanding).

MDF — Special Considerations

MDF is manufactured from compressed wood fibres and has two distinct surface types: the face (smooth, relatively low porosity) and the edges (highly porous cut fibres). The edges absorb primer like a sponge and will show texture through the final finish if not properly treated.

For MDF edges:

1. Sand edges with 80 grit to remove any machining marks
2. Apply 2 coats of MDF primer or neat PVA (exterior grade) to the edges only, sanding between coats with 180 grit
3. Prime the whole board as normal
4. Apply undercoat and topcoat as standard

Never use oil-based wood primer on MDF as it raises the surface fibres. Use a water-based acrylic primer or dedicated MDF primer.

Previously Painted Surfaces — Preparation Sequence

1. Clean — wash with sugar soap solution; rinse thoroughly; allow to dry completely
2. Assess adhesion — check for flaking or peeling; scrape back any loose paint to a firm edge
3. Degloss — sand all areas to be repainted with 120-150 grit; or apply liquid deglosser (e.g. Polycell Liquid Sander) with a cloth and allow to dull as directed
4. Fill — fill holes and dents with fine surface filler; sand flush when dry
5. Prime — prime any bare wood areas with appropriate primer; do not prime over sound existing paint
6. Undercoat — apply one coat of undercoat; this builds film and hides the sanding marks
7. Topcoat — two coats of finish

If the existing paint is sound (not flaking, not lifting) but is a very dark colour and you are applying a light finish, use a tinted undercoat to reduce the number of finish coats needed.

Exterior Window Frames

Window frames take the hardest environmental punishment. Oil-based gloss is still the preferred finish on exterior softwood window frames — it is harder and more water-resistant than water-based alternatives. Use a flexible sealant between the frame and surrounding masonry (not a rigid filler) — this joint must flex with building and thermal movement.

On hardwood frames (oak, meranti), use a stain or oil system if the timber is already stained; switching from stain to paint requires full stripping back to bare wood and starting from scratch with primer.

Frequently Asked Questions

Do I need oil-based primer or can I use water-based?

For softwood interior joinery, water-based acrylic wood primer is fine and has the advantage of quick drying (1-2 hours). For exterior softwood, or for highly resinous species like pine and larch, oil-based primer provides better penetration and resin sealing. For MDF, always use water-based or MDF-specific primer.

Can I skip the undercoat?

No. The undercoat is a critical part of the paint system — it provides film build, opacity, and a key coat between primer and topcoat. Skipping undercoat on woodwork leaves the finish thin, patchy, and prone to showing every imperfection. It also results in early wear on doors and heavily trafficked surfaces.

How long should I wait between coats on woodwork?

Oil-based products: primer 16-24h; undercoat 16-24h; topcoat recoat 16-24h. Water-based products: primer 1-2h; undercoat 2-4h; topcoat recoat 2-4h. In cold, damp conditions, double these times. If in doubt, touch-test for tackiness — the coat must be fully touch-dry before overcoating.

My gloss finish shows brush marks — how do I avoid this?

Brush marks in oil gloss typically result from: overworking the paint after it starts to set, or using a poor quality brush. Use a good quality natural bristle brush (Purdy, Hamilton, or similar). Lay off each section in long strokes with minimal pressure — do not go back over an area once the gloss has begun to tack. For a perfect finish on doors, apply with a short pile mini-roller and then lay off with a brush ('roll and tip' method).

Regulations & Standards

• BS EN 927 — paints and varnishes for exterior wood; test methods for durability

• COSHH Regulations 2002 — applies to solvent-based primers and oil-based paints; ensure adequate ventilation; review SDS

• VOC Regulations (The Paints Regulations 2012) — solvent-based woodwork paints must meet VOC limits; most major brands are compliant

• Building Regulations Approved Document B (Fire Safety) — intumescent paint required on fire-rated door sets; standard woodwork paint is not suitable on FR30/FR60 doors

• Dulux Trade Woodcare Technical Support — system guides for wood primer, undercoat, and topcoat

• Johnstone's Trade Woodwork Range — product specifications

• Rustins Technical Data — knotting solution and specialist wood prep products

• Guild of Master Craftsmen — Painting & Decorating — trade standards reference

• interior emulsion — emulsion selection once woodwork is complete

• paint coverage rates — estimating paint quantities for woodwork

• spraying vs rolling — spray application on doors and large woodwork areas

• first fix second fix — understanding when woodwork is installed and sequencing with painting