Filling and Sanding: Surface Preparation for a Flawless Finish

Quick Answer: Filling and sanding is the surface-preparation stage that turns a rough, marked wall or woodwork into a flat, sound, ready-to-paint surface — fill holes, cracks and dents with the right filler, let it cure, then sand flush and dust off. The professional rule is "preparation is 80% of the job": the paint can only ever be as flat as the surface beneath it. This work falls under BS 6150 (code of practice for painting of buildings), which makes sound, clean, made-good substrates a precondition of a durable decorative finish.

Summary

A perfect paint finish is made before the paint tin is opened. Filling and sanding is the unglamorous core of decorating — it is where a job is won or lost. Every nail hole, hairline crack, dented corner, proud nib of old paint and patch of flaky distemper telegraphs straight through the final coats, especially under modern matt emulsions and in raking light from a window. Customers run their hand over a skirting and look across a wall towards the light; that is exactly where poor prep shows.

The work matters because no paint, however expensive, hides texture — it follows it. Sheen makes it worse: the higher the sheen (satin, eggshell, gloss), the more mercilessly every defect is highlighted. Getting filling and sanding right is what lets a decorator charge a professional rate, because the finish genuinely looks different. It is also where time is spent: on a repaint, prep routinely takes longer than painting, and quoting a job means quoting the prep honestly rather than hoping two coats will cover the sins.

The common misconceptions are predictable. One: that you can fill a deep hole in a single pass — you can't reliably, because deep filler slumps and shrinks, so it needs building in layers. Two: that any filler will do — a flexible caulk is wrong for a sandable repair, and a hard cement filler is wrong for a fine skim over a crack. Three: that sanding is optional or can be done dry with no dust control — dry-sanding old paint can disturb lead-based coatings in pre-1992 properties and generates harmful dust, so identifying the surface and controlling dust is part of the job, not an afterthought. Four: that you sand once — high-quality work is fill, sand, spot-prime, look in raking light, then fill and sand again.

Key Facts

• Prep is most of the job — on repaints, filling and sanding commonly takes longer than the painting itself; quote it as real time.
• Match filler to defect — fine surface filler for hairlines and small dents, deep-gap/repair filler for holes, flexible filler/caulk for moving joints, wood filler for timber.
• Build deep fills in layers — fill deep holes in passes, letting each cure, to avoid slumping, shrinkage and cracking.
• Slightly proud, then sand flush — leave filler a touch proud of the surface so you can sand it back dead flat rather than leaving a hollow.
• Powder vs ready-mixed — powder fillers (mix to order) set harder, sand cleaner and shrink less; ready-mixed tubs are convenient for small jobs but can shrink more on deep fills.
• Grit progression — knock back hard filler with coarser paper (around 120 grit), then finish with finer (180–240 grit) for a paint-ready surface; finer still for gloss work.
• Sand with the light — work where raking light from a window crosses the surface so you see every defect; a hand-held lamp held flat to the wall reveals what overhead light hides.
• Dust control is mandatory — vacuum-assisted or wet/abrasive sanding controls fine dust; HSE requires dust to be controlled at source.
• Pre-1992 paint may contain lead — do not dry-sand old paintwork until you've considered lead; wet-sand and avoid heat/dust if in doubt.
• Spot-prime fillers — bare filler and timber are more absorbent than the surrounding paint; spot-prime or mist-coat so they don't "flash" (show as dull patches) under emulsion.
• Caulk last, after filling — fill and sand the faces first, then caulk the long trim-to-wall lines (caulk is tooled, not sanded).
• New plaster needs a mist coat — thin the first emulsion coat to seal absorbent new plaster before normal coats.
• Sound substrate first — scrape flaking paint, remove nibs, treat any damp/staining and ensure the surface is clean and dry before filling.

Quick Reference Table

Detailed Guidance

Reading the surface and making it sound

Before any filler, assess the surface. Scrape and strip flaking, blistered or unsound paint back to a firm edge; feather the edges so there is no hard step. Knock off nibs, paint runs and plaster splashes with a scraper or sanding block. Identify the substrate — sound plaster, new plaster, timber, previously glossed wood, filler patches — because each behaves differently under filler and paint. Check for and treat the cause of any damp, mould or stain first; filling and painting over an active problem just hides it until it bleeds back through. Only fill a clean, dry, sound surface.

Choosing and mixing the filler

Use the right filler for the defect. Fine surface fillers spread thin and sand to a feather edge — ideal for hairlines, small dents and finishing passes. Deep-gap/repair fillers are formulated to be built up without slumping — use them for holes and missing chunks, in layers. Flexible fillers and decorator's caulk handle moving joints where a rigid filler would crack. Wood filler suits timber, with exterior grades outside. Powder fillers mixed to a smooth, lump-free paste generally set harder, shrink less and sand cleaner than ready-mixed tubs, which is why they're the trade default for serious making-good; ready-mixed is fine for quick, shallow work. Mix only what you'll use before it goes off.

Applying filler — proud, layered, smooth

Press filler firmly into the defect with a flexible filling knife so it keys and there are no voids, then draw it off leaving the fill very slightly proud of the surrounding surface. Proud, not flush — you'll sand it back flat, and a fill left flush almost always sands into a shallow hollow. For deep holes, fill in two or more passes, letting each cure before the next, so it doesn't slump or crack. Feather the edges of the fill into the surrounding surface as you go to minimise sanding. Clean the knife between strokes.

Sanding back — grit, technique and light

Once the filler is properly cured, sand it flush. Start with a coarser grit (around 120) to knock hard filler back to the surface, then finish with finer paper (180–240, finer for gloss-grade work) to remove scratches and leave a paint-ready surface. Use a flat block or sanding board on flat areas so you don't dish the surface; sand by hand into mouldings and details. The single most important habit is to sand with raking light — position yourself so light from a window crosses the surface at a low angle, or run a lamp flat along the wall, so every ridge and hollow throws a shadow. What looks flat under overhead light is rarely flat under raking light.

Dust control, health and finishing prep

Sanding generates fine respirable dust; control it at source with a vacuum-assisted sander, dust extraction or wet/abrasive methods, and wear suitable RPE for the dust involved — this is an HSE requirement, not optional. In any property built or last decorated before 1992, treat old paint as potentially lead-based: do not dry-sand or burn it off without precautions; wet-sand or seek the right guidance. After sanding, dust the surface down thoroughly and wipe with a damp cloth or tack rag so no dust contaminates the paint. Spot-prime or mist-coat bare filler, timber and new plaster so they don't flash as dull patches through the finish, then check once more in raking light before painting.

Frequently Asked Questions

What grit sandpaper should I use before painting?

Knock back hard filler and rough surfaces with around 120 grit, then finish with 180–240 grit for a smooth, paint-ready surface. For gloss and high-sheen woodwork, go finer still and lightly key between coats. Coarser than this leaves scratches that show through; finer alone won't cut the filler back fast enough.

How long should filler dry before sanding?

Until it is fully cured — thin fine-surface fillers can be ready in under an hour, but deep fills need much longer, often several hours or overnight, and should be sanded only when hard right through. Sanding filler that's still soft underneath tears it out and leaves a hollow. Build deep fills in layers so each cures properly.

Why does my filler show through the paint?

Usually because bare filler is more absorbent than the surrounding painted surface, so it dries to a duller "flashed" patch — spot-prime or mist-coat the fill before painting. It can also show because it wasn't sanded flush (proud or hollow) or because the surrounding sheen highlights it. Prime fillers and check flatness in raking light.

Do I need to control dust when sanding?

Yes. Sanding dust is a respiratory hazard, and HSE requires it to be controlled at source — use a vacuum-assisted sander, extraction or wet sanding and appropriate RPE. In any pre-1992 property, old paint may contain lead, so avoid dry sanding and dust generation and take the proper precautions.

Regulations & Standards

• BS 6150 — Code of practice for painting of buildings; covers cleaning down, making good, filling and surface preparation as a precondition of a sound finish.

• Control of Substances Hazardous to Health Regulations 2002 (COSHH) — governs control of harmful dust (including sanding dust) at source.

• Control of Lead at Work Regulations 2002 (CLAW) — applies where old (typically pre-1992) lead-based paint is disturbed by sanding or stripping.

• Manufacturer technical data sheets — authoritative for filler cure times, depth limits and overcoating.

• BSI – BS 6150 (Painting of buildings – Code of practice) — UK code of practice for surface preparation and decoration.

• HSE – COSHH and controlling construction dust — dust control duties.

• HSE – Lead and work with lead — precautions when disturbing old lead-based paint.

• Painting and Decorating Association (PDA) — UK trade body preparation guidance.

• caulking before painting — caulking trim joints after filling and sanding

• woodwork prep — preparing and keying timber and previously glossed surfaces

• interior emulsion — mist coats and applying emulsion over prepared surfaces

• lining paper before decorating — when lining beats filling on poor walls