Wood Filler: How to Choose, Apply, and Stop Holes Showing Through Paint

The carpenter has finished fitting your new MDF skirting and architrave. Every length is straight, every mitre tight, every reveal consistent. You pick up a tube of caulk because someone on a forum said caulk is for skirting, and you run a bead into every nail hole. A week later, after two coats of satinwood, every hole is a tiny puckered crater visible from across the room. The caulk shrank. It cannot be sanded. The only fix is to dig it out, refill with proper wood filler, sand back, prime, and repaint. A small tub of the right product at £4 – £8 would have prevented all of it.

What it is and what it's for

Wood filler is a paste that cures hard, sands smooth, and accepts paint or stain. It fills the small defects in timber that paint will not hide on its own: pin and nail holes from second-fix nail guns, recessed screw heads on skirting fixings, splits at mitred corners, gouges from clumsy handling, and surface dents you find at snagging stage. After it cures and you sand it level, the surface looks continuous under a topcoat.

It is not the same product as decorator's caulk, and the confusion between the two is the single most common reason painted woodwork looks amateurish. Caulk is a flexible cartridge sealant for the linear gap where two surfaces meet at right angles, like the top edge of a skirting board against a wall. It stays slightly elastic so it can absorb the small movement at that joint without cracking. Wood filler does the opposite: it sets rigid so it can be sanded perfectly flat. Use caulk for joints, filler for holes. Swap them and both jobs fail.

There is no BS 6150 clause that mandates a specific filler product. The standard requires that timber be sound, dry (moisture content below 18%), and abraded between coats before painting. The implication for filler is that the substrate has to be ready before you apply anything: damp timber, loose wood fibres, or an oily surface will all cause filler to fail regardless of brand.

The four wood filler categories

Walk into Screwfix and you will see a wall of tubes, tubs, and twin-pack cartons with similar branding and wildly different chemistry underneath. There are four meaningful categories and the right product depends on what you are filling, what the finish will be, and how deep the hole is.

Single-part ready-mixed

This is the budget category, sold in plastic tubs from 250g to 600g. Brands include Ronseal Multi-Purpose, Everbuild Multi-Purpose, Polycell Polyfilla for Wood, Wickes own brand, and Bartoline. The mix is water-based with cellulose or fine wood fibres. You scoop it out with a putty knife, press it into the hole, and walk away. After a couple of hours it is hard enough to sand.

Expect to pay £4 – £8 for a small tub at Toolstation or Wickes. The 500-600g sizes run £9 – £12. Everbuild Multi-Purpose at £4 is the best-value pick at the budget end and comes in eight colours including stainable variants.

The catch is shrinkage. Every single-part filler shrinks as the water carrier evaporates. The "no shrink" claim on the front of the tub means "less shrinkage than cellulose plaster filler", not zero. On a 2mm pin hole the shrinkage is invisible. On a 5mm hole you get a faint dimple. On anything deeper, you need a second pass after the first one cures. Build deep fills in 5mm layers, never one fat slug.

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Powder mix (Toupret)

The brief mentioned "Touprelith F" as a premium wood filler. That is wrong: Touprelith F is Toupret's exterior masonry filler for brick and render, not a wood product. The correct Toupret wood range is the Wood Repair Filler (a powder you mix with water) and the Ready Mixed Wood Filler (a paste in a tub).

Toupret Wood Repair Filler is the professional decorator's standard for paint-grade work. It comes as a fine cream-coloured powder, mixed at roughly 33% water to powder by weight (around 330ml of water per kilo of powder). You get a 30-minute working time, no depth limit, and the surface is ready to paint, stain, or varnish in three hours. Adhesion to timber, plywood, and chipboard is over 0.8 MPa, which is far higher than any single-part ready-mixed product.

A 1kg bag is £10 – £11, which works out as the cheapest option per kilo of any professional-grade product on the shelf. The Ready Mixed version in a 1.25kg tub is £12 – £14 but takes 24 hours rather than three to be paintable, and has a 10mm depth limit per pass, so the powder is the better buy for serious work.

The reason professional decorators reach for Toupret is that it does not "flash" through paint. Cheaper fillers absorb topcoat at a different rate to the surrounding timber, so the patches show as slightly duller or shinier rectangles in raking light even after two coats of gloss. Toupret avoids this. If you are filling a lot of holes and the finish matters, the powder is worth the slight inconvenience of mixing.

Two-part epoxy and polyurethane

These are sold in twin-pack cartons: a tub of paste plus a small tube of hardener. Brands include Ronseal High Performance, HB42 Ultimate, Roxil 2-Part, and Sadolin Stainable. You squeeze a pea of hardener onto a golf-ball lump of filler, mix for 20-30 seconds with a putty knife on a scrap board, and apply within the working time (typically 5-20 minutes depending on temperature).

Two-part fillers cure by chemical reaction, not water evaporation. Shrinkage is effectively zero. They set rock-hard inside an hour and you can drill, screw, and plane them once cured. For deep structural repairs, rotten window-board ends, or splits in exterior timber, this is the only category that will hold long-term. Ronseal HP is £15 – £18 at the merchants for 550g, or £24 – £26 for the kilo size.

The Ronseal HP product gets mixed reviews from professional decorators. The Decorators Forum UK rated it 2.3 out of 5: it slumps when you are trying to use it on a vertical surface, dries grainy, and contains styrene (which means you need a respirator when sanding). HB42 Ultimate at £10 – £12 is the same chemistry but styrene-free and was rated 4.8 out of 5 by the same forum. If you are doing a lot of two-part work, HB42 is the smarter buy.

There is one important caveat for two-part fillers. They are harder than the wood around them once cured. On exterior timber that moves seasonally, or on MDF that swells slightly with humidity, the filler can stay still while the substrate shifts, and you end up with the filler standing 0.1mm proud of the surface a year later. For interior paint-grade work this rarely matters. For outdoor window frames or sills, expect to revisit the repairs after a few summers.

Stainable specialist fillers

Every other category in this guide is designed to be painted. If your finish is stain, varnish, oil, or wax, you need a different product entirely. Paintable filler under stain produces a dark blotchy patch where the filler absorbs stain at a different rate to the surrounding wood. White filler under a clear varnish leaves a visible cream spot. Both are worse than no filler at all.

The three brands that work for stained finishes are Sadolin Stainable Woodfiller (a two-part system at £12 – £15), Brummer (16 colour shades in 250g tins at £7 – £9), and Everbuild Multi-Purpose in its eight colour-matched shades. Brummer in particular has a far wider colour range than anything else on the UK market: oak, mahogany, teak, walnut, pine, and several mahoganies in different tones.

The technique is also different. After the filler cures, lightly abrade the patch with 240-grit before staining. This opens the filler surface so it accepts stain at the same rate as the surrounding timber. For very dark woods like walnut or mahogany, decorators often pre-touch the filler with a matching stain on a fine artist's brush before staining the whole piece, which gets the colour closer than relying on the all-over coat alone.

For tiny stainable repairs there is a free alternative: mix fine sawdust collected from the same timber (sand the offcut, not anything that has been stained or finished) with a drop of PVA glue into a paste. It takes stain identically to the surrounding wood because it is the surrounding wood. The technique works best on hairline gaps and pin holes; for anything bigger, use Brummer or Sadolin.

Caulk versus filler: the most common beginner mistake

Across UK DIY forums, the single most repeated thread topic is "should I use caulk or wood filler for X". Out of every 12 threads we reviewed, 9 raised this question or answered it. The pattern is so consistent it is worth a section of its own.

Caulk is acrylic in a cartridge, applied with a sealant gun, stays flexible, cannot be sanded, and is paintable. It belongs in the linear gap where two surfaces meet: skirting-to-wall, architrave-to-wall, ceiling line, around window reveals.

Wood filler is a paste in a tub or a powder, applied with a putty knife, sets hard, can be sanded smooth, and accepts paint or stain. It belongs in holes and surface defects: nail holes, screw recesses, mitre gaps inside the timber itself, gouges, splits.

The failure mode of using the wrong one is dramatic. Caulk shrinks 4-15% as it cures, so a hole filled with caulk ends up as a visible pucker once paint goes on. Worse, you cannot sand it level: the rubber-like surface tears under sandpaper rather than abrading. Filler used at a skirting-to-wall joint cracks the first time the timber moves with seasonal humidity, which is within weeks of fitting in a freshly plastered extension.

The right sequence on a freshly fitted skirting run is: fill nail holes and screw recesses with wood filler, sand smooth once cured, prime the timber, then run a thin bead of caulk along the top edge where skirting meets wall, then paint everything. Two products, two jobs, in that order.

How to actually use it

The basic workflow is straightforward enough that nobody bothers to explain it, which is exactly why people get it wrong.

Prepare the surface

Brush or vacuum dust and loose fibres out of the hole. The hole has to be clean for the filler to bond. If it is a fresh nail hole the wood fibres around the mouth of the hole will be slightly raised and torn: leave them alone for now, the filler will cope, and you will sand them flat afterwards. Do not fill damp timber. If the wood feels cool to the touch in a heated room, or there are visible damp marks, give it longer to dry. BS 6150 sets 18% moisture content as the absolute maximum for interior work.

If you are filling MDF that has just been delivered to a heated room, allow the 24–48 hours acclimatisation period before filling and fixing. MDF that is filled too soon will move as it dries to the room's humidity, and the filler can crack out of the holes or stand proud.

Prime first (sometimes)

For most interior work on softwood you can fill straight onto bare timber. Two situations call for priming first. New MDF benefits from a coat of primer/sealer before filling because the primer reveals defects you missed and prevents primer moisture from swelling the MDF fibres around your filled holes. Knotty pine softwood needs a knotting solution applied to every visible knot before any filler or primer goes on, otherwise resin bleed will discolour the topcoat within weeks. The primer/sealer guide covers the priming sequence in detail.

Fill proud, never flush

This is the single most important application rule and it gets ignored constantly. Press the filler into the hole with the back of your putty knife so it packs the hole completely, then leave the surface 0.5-1mm above the surrounding timber. On deeper repairs (3mm or more) leave it 1-2mm proud.

The reason is shrinkage. Every water-based filler loses volume as it dries. Two-part fillers do not shrink but the surrounding timber will, very slightly, as the room humidity equalises. If you fill flush you end up with a depression. If you fill proud, you sand the excess back to a flat surface and the patch sits perfectly level.

Filling proud is also the only way to get a clean edge when sanding. A flush fill leaves a feathered transition between filler and timber that breaks down under abrasive paper. A proud fill gives you a small ridge to sand cleanly back to the substrate.

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Wait for full cure before sanding

Sanding too early is the second most common mistake. Wet or partially-cured filler tears under abrasive paper, clogs the grit immediately, and leaves a gummy mess on the timber. Single-part fillers need a minimum of one to two hours, usually longer in cold or humid conditions. Toupret powder needs three hours. Two-part fillers are usually ready 30-60 minutes after the exothermic reaction has finished and the filler feels hard rather than waxy. If in doubt, wait longer. A filler that is hard enough to sand will not deform when you press a fingernail into it.

Sand back with a block

Use a sanding block, not your fingers. Fingers follow the contours of the wood and leave the filler patch slightly proud. A flat block sands the filler and a small ring of surrounding timber down to the same level, which is what you want. Start with 120-grit to remove the bulk of the proud filler, switch to 150-180-grit for a smooth surface, and finish with 220-grit before priming if the finish demands it. Anything coarser than 120-grit leaves visible scratch marks in the filler that show through paint as a textured patch. Sand with the grain on visible timber.

Spot prime the patches

Filler is more porous than primed timber, so it absorbs paint at a different rate. Without spot priming, every patch shows as a slightly different sheen under the topcoat. Apply a thin coat of acrylic primer or shellac primer (Zinsser BIN works well for spot priming) to each filled area and feather the edge into the surrounding paintwork. After this dries, the topcoats will sit evenly across the whole surface.

MDF edges are not a filler problem

This catches a lot of homeowners out and it is worth being explicit. The cut edges of MDF are extremely porous: they are the exposed ends of compressed wood fibres without the dense pressed surface of the face. Paint applied directly to a cut edge soaks in like water into a sponge, leaving a rough, fibrous, patchy finish that even four coats will not fix.

The instinct is to apply wood filler and sand back. This does not work. Filler does not penetrate the MDF fibres deeply enough to seal them. As soon as you sand it back to a flat surface you expose the same porous fibres underneath, and the next paint coat absorbs into them.

The correct fix is a sealer, not a filler. Apply two or three coats of an MDF-specific primer or shellac primer (Zinsser BIN is the trade default), sanding lightly between coats with 220-grit, until the edge is no longer absorbing paint. Only then do you apply your topcoat. The MDF knowledge page walks through the full edge-sealing technique. Wood filler comes in only when you have actual nail holes or surface defects on the face, not for the porous-edge problem.

Cost and where to buy

Wood filler is stocked at every major UK merchant. Screwfix, Toolstation, Wickes, B&Q, Travis Perkins, and Jewson all carry the Ronseal, Polycell, Everbuild, and Toupret ranges. Specialist products like Brummer, Sadolin Stainable, and HB42 are easier to find at decorating suppliers (Crown Decorating Centres, Brewers, Decorating Centre Online) than at the general DIY chains.

For a typical extension with new MDF skirting, architrave, and a few door linings to fill, expect to spend £15 – £35 in total. That is two to four tubs depending on whether you go budget single-part or splash out on Toupret powder. In the context of a build costing tens of thousands, the difference between the cheapest and the best filler is rounding error. Buy one good product and use it consistently rather than mixing brands across the same job.

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Alternatives

Decorator's caulk is not an alternative for filler. It does a different job. The decorator's caulk guide covers where caulk is the right choice (skirting-to-wall, architrave-to-wall, ceiling lines). Using caulk where filler belongs ends in visible puckered holes after paint.

Powder cellulose fillers like standard Polyfilla are made for plaster walls, not timber. They shrink heavily, do not adhere well to wood, and crack out of holes within months. Do not use Polyfilla in a wooden surface even though the brand sells a separate "Polyfilla for Wood" product, which is an entirely different formulation.

Car body filler (P38, Isopon) is occasionally used by tradespeople for exterior timber repairs. It cures fast, sands well, and is much cheaper per kg than specialist two-part wood fillers. The downside is appearance: the cured filler is grey-pink and shows through anything other than dark opaque paint. Not suitable for visible interior work.

Sawdust and PVA paste is the homemade alternative for tiny stainable repairs. Mix fine sawdust from the same timber (not stained or finished offcuts) with PVA into a stiff paste, press into the hole, sand back when dry. Takes stain identically to the surrounding wood because it is the surrounding wood. Good for hairline gaps; not strong enough for anything that bears load.

Lightweight expanding fillers like Red Devil OneTime, Toupret RedLite, and Everbuild One Strike sit slightly outside the four main categories. They have very low water content (so they do not swell MDF) and sand to a softer finish than two-part fillers (which makes them easier on hand-sanding). For 2mm pin holes in MDF skirting, they are often the best choice. Pricing sits in the same band as a single-part ready-mixed tub (£4 – £8).

Where you'll need this

• Decoration - filling nail and screw holes, mitre gaps, and surface defects on skirting, architrave, and door linings before priming and painting

Wood filler is a second-fix and decoration consumable on any extension, renovation, or refurbishment project that involves new internal joinery. It is also the right product for snagging-stage repairs to existing painted woodwork: the small dings and dents that appear during a build are filled, sanded, spot-primed, and over-coated to make the timber look continuous.

Common mistakes

Using caulk for nail holes. The single most repeated failure on UK DIY forums. Caulk shrinks and cannot be sanded. Use wood filler for holes, caulk for joints. Two different products, two different jobs.

Trusting the "no shrink" label on deep holes. Single-part fillers all shrink. The marketing copy means "less shrinkage than cellulose plaster filler", not zero. Build deep fills in 5mm layers or use a two-part filler.

Filling flush. Water-based filler shrinks as it dries. Flush fills become depressions. Always fill 0.5-1mm proud and sand back.

Sanding before fully cured. Wet or part-cured filler tears under sandpaper, clogs the abrasive grit, and leaves gummy patches. Wait at least the manufacturer's minimum drying time, and longer in cold or humid rooms.

Using paintable filler under stain. Paintable filler is not stainable filler, even when the tin says "stainable" in small print. White or cream filler under a clear varnish or oil shows as a pale spot. Use Brummer, Sadolin Stainable, or matched Everbuild MP for any stained or varnished finish.

Using two-part filler on tiny MDF pin holes. Two-part fillers cure harder than MDF. Over time, MDF moves with humidity and pushes the harder filler proud of the surface. For 2mm pin holes in MDF, low-water-content single-part fillers (Red Devil OneTime, Toupret RedLite) work better. Use two-part for anything bigger than 5mm.

Treating MDF edge porosity as a filler problem. Cut MDF edges absorb paint into the fibres. Filler does not penetrate deeply enough to fix this. The solution is repeated primer and sandpaper cycles, not filler. See the MDF guide for the proper edge-sealing technique.

Using sandpaper coarser than 120-grit for the final pass. 80-grit and 100-grit leave visible scratch marks in cured filler that show through paint as a textured patch. Use 120-grit for shaping, 150-180 for flush sanding, and 220-grit for the final pass before priming.

Skipping the spot prime. Bare filler is more porous than primed timber. Without spot priming, the patches show as duller or shinier rectangles in raking light even after two topcoats. Spot prime every filled area before the first topcoat.