How to Repair and Fill Skirting Boards, Architraves, and Timber Trim Before Painting product guide

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Why Trim Repair Is a Two-Product Job — Not One

Walk into any hardware store and ask for something to fix your skirting boards before painting, and you'll likely be handed a single tub of filler. That's the first mistake most DIYers make — and it's the root cause of the cracked, peeling, or falling-out repairs they'll be back to fix six months later.

Skirting boards, door architraves, and window trims present a repair challenge that is fundamentally different from patching a wall or filling a crack in plasterboard. These timber elements sit at a junction between two dissimilar materials — timber and plaster or masonry — each of which moves at a different rate in response to temperature and humidity changes. Timber is a natural material that never stops reacting to its environment: when humidity rises, boards expand slightly, and when the air dries out, they contract.

Skirting boards and floorboards, being natural timber, can shrink from day-to-day exposure to the climate inside a home, such as heating and air conditioning, which reduces humidity.

This is why the gap between the skirting board and the wall is a movement joint, not a static crack. Fill it with a rigid filler, and it will crack and fall out within one season — guaranteed.

At the same time, the face of the timber itself — the chips, dents, nail holes, and gouges that accumulate over years of use — requires a completely different solution: a hard-setting wood filler that can be sanded perfectly flush and hold up under the unforgiving scrutiny of a gloss topcoat.

Treating these as two separate tasks, requiring two separate products, is the insight that separates a professional-quality trim repair from a failed one. This guide walks through both workflows in full, step by step. (For help choosing between the full range of Selleys fillers for different repair scenarios, see our guide on Wall Filler vs. Wood Filler vs. Gap Filler: Which Selleys Product Do You Actually Need?)

Understanding the Two Repair Zones on Timber Trim

Before picking up any product, it helps to diagnose your trim correctly. Every piece of skirting board, architrave, or window trim has two distinct repair zones:

Confusing these zones is the single most common cause of trim repair failure. (See our companion article Why Your Filler Keeps Cracking, Shrinking, or Falling Out — and How to Fix It for Good for a full breakdown of failure modes and their causes.)

Zone 1: Filling the Gap Between Timber Trim and the Wall

Why This Gap Exists — and Why It Will Return

While accommodating thermal movement isn't generally considered necessary, it is recommended that designers of wood-frame buildings account for expansion during construction due to increased moisture exposure. In practice, this means the gap between a skirting board and the wall plaster is an engineered feature, not a defect. If the correct expansion gap has not been used, boards can expand and push the next board along, which can cause walls to be pushed out of line and cracks to appear on internal walls.

This is why a rigid filler — no matter how carefully applied — will eventually crack at this junction. The building is moving; the filler is not.

The Right Product: Selleys No More Gaps Interior Multipurpose

Selleys No More Gaps Interior Multipurpose is an easy-to-use gap filler ideal for filling gaps and cracks around skirting boards, cornices, and inside window and door frames. The water-based formulation has been designed to cope with building movement to ensure a long-lasting and crack-resistant finish.

The key performance specification here is elongation. Selleys No More Gaps Interior Multipurpose Filler stretches over 300% and allows up to ±15% movement to accommodate building shifts. This is what makes it suited to a movement joint — it flexes rather than fractures. It also provides excellent adhesion to materials like timber, masonry, plasterboard, ceramic tiles, glass, and Colorbond.

For gaps up to a specific size, the product is self-contained. Ideal for filling gaps up to 25mm wide and 12mm deep, this low-shrinkage filler adheres strongly to a wide range of building materials, including timber, brick, plasterboard, and aluminium.

Step-by-Step: Applying No More Gaps Interior to the Wall-Timber Junction

What you'll need:

• Selleys No More Gaps Interior Multipurpose (cartridge or squeeze tube)
• Caulking gun (for cartridge format)
• Painter's masking tape
• Damp cloth or wet finger
• Fine-mist spray bottle

Steps:

1. Clean the junction. Remove any old, cracked filler using a scraper or utility knife. Wipe the area clean of dust, grease, and debris. Allow to dry fully.

2. Apply masking tape. Tape both sides of the gap before applying the product. This ensures a neat finish and makes it easier to clean up.

3. Prime the cartridge. When first applying product, extrude a small amount onto a piece of scrap material to ensure a smooth, continuous flow of gap filler and a perfect finish.

4. Apply the filler. Push Selleys No More Gaps Interior Multipurpose Filler firmly into gaps around skirting boards, doors, or windows using a filling knife. Run the bead in a single, continuous pass along the full length of the junction.

5. Tool the bead. Smooth the filler and wipe away any excess with a damp cloth. A wet finger drawn along the bead in one smooth pass will create a concave profile that sits neatly at the junction. Note: if you are going to wipe off the excess gap filler with a wet finger, ensure you wet your finger with water and not saliva, as saliva contains bacteria and this will increase the chances of mould growing in your gap filler.

6. Remove tape immediately. Pull masking tape away at a 45-degree angle before the filler skins over.

7. Allow skin time before painting. Let the filler skin over for about 30 minutes before painting. In humid or cool conditions, allow longer.

8. Inspect and re-fill as needed. Check for new cracks caused by building movement and refill as needed.

Pro tip: In rooms with significant temperature swings — such as unheated rooms in cold climates or sun-exposed north-facing rooms — consider running a second, thin pass of No More Gaps over the first once it has cured. This builds a slightly thicker flexible film that provides greater movement tolerance.

Zone 2: Repairing Chips, Dents, and Nail Holes in the Timber Face

Why the Timber Face Needs a Different Product Entirely

Once the junction is sealed, attention turns to the timber surface itself. Here, the requirements are the opposite of Zone 1. You need a filler that:

• Sets hard so it can be sanded perfectly flush
• Holds up under gloss paint without telegraphing through
• Bonds tenaciously to timber (not plaster or masonry)
• Can be feathered to a zero-edge so the repair is invisible under raking light

Flexible gap fillers are entirely wrong for this application. Selleys Plastic Wood is not recommended for use where structural movement may occur — use Selleys No More Gaps if flexibility is required. The reverse is equally true: No More Gaps should not be used on the flat timber face, because its flexibility prevents it from sanding to a crisp, flush finish.

Choosing Between Two Hard-Setting Options

Selleys offers two distinct products for timber face repairs, each suited to slightly different scenarios:

Selleys No More Cracks Doors & Trims

Selleys No More Cracks Doors & Trims ready-to-use filler can be used inside or outside with gloss paints and applications such as doors, windows, architraves, trims, and skirting boards. Its standout feature is its finish quality under high-sheen coatings: it gives a mirror-like, durable finish with superior gloss retention and has excellent adhesion to timber, plaster, concrete, mortar, and primed metal. This makes it the go-to choice when the final coat will be a semi-gloss or full-gloss enamel — the standard finish for interior timber trim in Australian homes.

Selleys Plastic Wood

Selleys Plastic Wood is a fast-setting, solvent-based wood filling cement containing natural wood fibres, delivering a hard, maple-coloured finish that blends seamlessly with timber surfaces. It sets quickly and dries to a finish that can be treated in every respect like real wood — sanded, nailed, planed, stained, varnished, painted, or lacquered. This is the better choice for deeper repairs, for timber that will be stained rather than painted, or where you need a filler that truly mimics the working properties of wood.

Step-by-Step: Repairing the Timber Face with No More Cracks Doors & Trims

What you'll need:

• Selleys No More Cracks Doors & Trims (tub)
• Filling blade or flexible scraper
• 120-grit sandpaper (for initial shaping)
• 180–220-grit sandpaper (for fine sanding)
• Tack cloth or damp rag
• Water-based undercoat
• Dust mask

Steps:

1. Prepare the surface. Ensure the surface to be filled is clean and dry with no loose or powdery material. If the existing paint is glossy, lightly sand the repair area with 120-grit sandpaper to give the filler a mechanical key. Wipe clean with a tack cloth.

2. Apply the filler. Use a filling blade or scraper to firmly press the filler in and scrape off excess to minimise sanding. For nail holes and small dents, a single pass with the blade held at about 30 degrees to the surface will pack the filler in and wipe the excess clean in one motion.

3. Overfill slightly for deeper repairs. For chips or gouges deeper than 2mm, slightly overfill the repair so the filler sits fractionally proud of the surrounding surface. This accounts for minor shrinkage during drying.

4. Limit layer depth. Apply with a putty knife in layers no deeper than 6mm, slightly overfilling to allow for sanding back to a flush finish. For deeper damage, build up in two or more layers, allowing each to dry before applying the next.

5. Allow to dry. Allow to dry for 2–3 hours, depending on humidity, then sand with fine sandpaper. In high-humidity conditions or for thicker applications, allow additional drying time before sanding.

6. Sand to flush. Wearing a dust mask, lightly sand away any leftover excess with sandpaper. Begin with 120-grit to knock back any proud material, then finish with 180–220-grit to blend the repair edge into the surrounding timber.

7. Apply a second coat if needed. For a perfect finish, apply another thin coat of filler, allow to dry, before sanding lightly with fine sandpaper. This second coat fills any micro-shrinkage in the first and produces the level surface needed for gloss paint.

8. Prime before painting. For best results, apply a water-based undercoat after any sanding. This step is non-negotiable for gloss finishes: an unprimed filler patch will absorb the topcoat unevenly, leaving a visible "halo" of dull paint around the repair — sometimes called "picture framing."

9. Clean up before the filler dries. Uncured product may be removed with water.

Achieving a Factory-Smooth Finish Under Gloss Paint

Gloss paint is the most demanding topcoat you can apply to timber trim. Unlike flat or low-sheen paints, which scatter light and hide minor surface irregularities, gloss paint reflects light directly — making every imperfection visible. A repair that looks flawless under flat paint can show as a distinct shadow or ridge under gloss.

The following finishing protocol is specifically designed for gloss-painted skirting boards and architraves:

The Two-Coat Fill, Two-Sand Method

1. First fill — Pack the repair and scrape flush. Allow full drying time.
2. First sand — Use 120-grit to level any proud material. Wipe clean.
3. Second fill — Apply a thin skim coat over the repair. This fills micro-porosity and any shrinkage.
4. Second sand — Use 180-grit, then finish with 220-grit. The surface should feel glassy smooth to the fingertip.
5. Prime — Apply one coat of water-based undercoat. Allow to dry per manufacturer's directions.
6. Inspect under raking light — Hold a torch or work light at a low, oblique angle to the surface. Any remaining imperfections will cast a shadow at this angle and can be spot-filled before the topcoat.
7. Light sand — A final pass with 240-grit over the primed surface removes any dust nibs or brush marks from the undercoat.
8. Apply gloss topcoat.

For Selleys No More Cracks Wood Filler, Selleys' own technical guidance confirms this two-pass approach: for a perfect finish and to minimise sanding, slightly overfill and sand back with fine (>200 grit) sandpaper upon drying, then apply a final thin coat of filler and allow to dry before sanding lightly with very fine (>400 grit) sandpaper.

Common Mistakes That Ruin Trim Repairs

Key Takeaways

• Trim repair is always a two-product job. The junction between timber and wall is a movement joint requiring a flexible gap filler (Selleys No More Gaps Interior). The timber face requires a hard-setting wood filler (Selleys No More Cracks Doors & Trims or Plastic Wood). Using one product for both zones is the leading cause of trim repair failure.
• Flexibility is the non-negotiable specification for junction gaps. Selleys No More Gaps Interior stretches over 300% and allows up to ±15% movement — essential for accommodating the seasonal expansion and contraction of timber against plaster.
• Gloss paint demands a two-coat fill and raking-light inspection. Because gloss reflects light directly, any surface irregularity becomes visible. The two-fill, two-sand method with a primed undercoat is the only reliable way to achieve an invisible repair under high-sheen finishes.
• Primer is not optional. Applying gloss topcoat directly over bare filler creates an uneven sheen that draws the eye to the repair rather than hiding it. A water-based undercoat is always the correct final step before the topcoat.
• Layer depth matters for hard-setting fillers. Apply in layers no deeper than 6mm , slightly overfilling to allow for sanding back. Deeper repairs built in a single pass are prone to surface cracking as the filler dries and shrinks.

Conclusion

Repairing skirting boards, architraves, and window trims to a paint-ready standard is one of those tasks where the right diagnosis makes all the difference. The moment you recognise that every piece of timber trim has two distinct repair zones — the junction and the face — and that each requires a fundamentally different product, the entire workflow becomes logical and predictable.

Selleys No More Gaps Interior handles the movement-tolerant sealing that the wall-timber junction demands, while Selleys No More Cracks Doors & Trims or Plastic Wood delivers the hard, sandable, gloss-ready surface that the timber face requires. Neither product can do the other's job.

For more on selecting the right Selleys product for your specific repair scenario, see our guide on Wall Filler vs. Wood Filler vs. Gap Filler: Which Selleys Product Do You Actually Need? If your trim has suffered more serious damage — rot, splitting, or structural deterioration — the techniques in How to Repair Rotted, Damaged, or Split Exterior Timber with Selleys Wood Fillers cover the higher-stakes repair process in full. And if a previous repair has already failed, Why Your Filler Keeps Cracking, Shrinking, or Falling Out will help you diagnose the root cause before you start again.

References

• Selleys Australia. "No More Gaps Interior Multipurpose." Selleys.com.au, 2024. https://www.selleys.com.au/products/putty-and-fillers/flexible-gap-fillers/selleys-no-more-gaps-interior-multipurpose/

• Selleys Australia. "No More Cracks Doors & Trims." Selleys.com.au, 2024. https://www.selleys.com.au/products/putty-and-fillers/wall-filler/selleys-no-more-cracks-doors-and-trims/

• Selleys Australia. "Plastic Wood." Selleys.com.au, 2024. https://www.selleys.com.au/products/putty-and-fillers/rigid-fillers/selleys-plastic-wood/

• Selleys Australia. "Filling Cracks in Doors and Trims." Selleys.com.au, 2024. https://www.selleys.com.au/home-improvement/living-areas/filling-cracks-in-doors-and-trims/

• WoodWorks | Wood Products Council. "Do Wood-Frame Buildings Need to Account for Thermal Movement?" WoodWorks.org, 2022. https://www.woodworks.org/resources/do-wood-frame-buildings-need-to-account-for-thermal-movement/

• Mortlock Timber. "Causes and Solutions for Timber Movement." Mortlock.com.au, 2026. https://www.mortlock.com.au/learning/understanding-timber-movement/

• Heartwood Floors. "Do You Really Need an Expansion Gap for Engineered Timber Flooring?" HeartwoodFloors.com.au, 2025. https://heartwoodfloors.com.au/blog/why-expansion-gaps-matter-in-engineered-timber-flooring-installation/

• Intrim Mouldings. "Do You Leave a Gap Between Skirting Boards and Floor Boards?" InTrimMouldings.com.au, 2020. https://intrimmouldings.com.au/do-you-leave-a-gap-between-skirting-boards-and-floor-boards/