Why Your Filler Keeps Cracking, Shrinking, or Falling Out — and How to Fix It for Good product guide

AI Summary

Product: Selleys Spakfilla Rapid, Selleys Spakfilla Heavy Duty, Selleys No More Gaps Multipurpose Brand: Selleys Category: Wall fillers, wood fillers, and flexible gap sealants Primary Use: Repairing cracks, holes, and gaps in interior and exterior surfaces including plaster, plasterboard, timber, masonry, and movement joints

Quick Facts

Best For: DIYers diagnosing and fixing failed filler repairs on walls, timber trim, and movement joints
Key Benefit: Matching the correct product to the substrate and joint type eliminates repeat repair failures
Form Factor: Ready-mixed paste (Spakfilla Rapid and Heavy Duty); flexible acrylic sealant (No More Gaps)
Application Method: Apply in 6mm layers with scraper or putty knife; allow full cure before sanding and painting

Common Questions This Guide Answers

Why does my filler keep cracking along the same line? → Rigid filler is being used at a movement joint; switch to Selleys No More Gaps Multipurpose, which has greater than 300% elongation and up to ±15% movement capability
How long should I wait before painting over filler? → Spakfilla Rapid can be painted with water-based paint once completely set (approximately 30 minutes for thin fills); oil-based paint requires 24 hours; painting before full cure traps moisture and causes cracking under the paint film
Why does my filler lift cleanly off the surface? → Insufficient surface preparation — filler cannot bond to dust, grease, loose paint, or moisture; remove all loose material, degrease with Selleys Sugar Soap, and sand glossy surfaces with 80–120 grit before applying filler

Why Your Filler Keeps Cracking, Shrinking, or Falling Out — and How to Fix It for Good

Few things are more frustrating on a DIY project than watching a repair go wrong. A crack reappears along the same line within weeks. The filler shrinks and sinks below the surface. Or the whole patch falls out cleanly, as though it never bonded at all. These outcomes aren't random. Every cracking, shrinking, or delaminating repair has a diagnosable root cause — and in almost every case, it comes down to one of seven predictable, preventable mistakes.

This guide diagnoses each failure mode, explains the mechanism behind it, and maps every problem to the correct Selleys product and technique for a lasting result. If you've already had a repair fail, this guide tells you exactly how to strip it back and do it right the second time.

Why Filler Fails: The Fundamental Principle

Before diagnosing specific failure modes, understand one governing principle: filler is only as good as the system it sits within. The substrate, surface preparation, product selection, application depth, cure time, and topcoat all interact. A breakdown in any one element compromises the whole repair. No matter how advanced your filler is, it won't perform if applied over contaminated, slick, or unstable surfaces.

This is why generic advice — "just fill and paint" — produces so many repeat repairs. The seven failure modes below each represent a break in the system.

Failure Mode 1: Wrong Product for the Substrate

The symptom: The filler looks fine initially, then cracks, crumbles, or detaches from the surface within weeks or months.

The cause: Not all fillers bond equally to all substrates. A wall filler applied to bare timber won't penetrate wood grain the way a purpose-formulated wood filler does. A general-purpose filler used on a surface that moves — like the joint between a skirting board and a plaster wall — will crack because it has no elasticity to accommodate that movement.

The fix: Match the product to the substrate and the repair type. Get this right, and you get it right the first time.

For plaster, plasterboard, brick, masonry, and primed metal where no structural movement occurs: Selleys Spakfilla Heavy Duty is a multi-purpose, tough and durable, light grey ready-mixed filler suitable for filling holes, dents, cracks, or chips in interior and exterior applications.
For wallboard, wood, plaster, and plasterboard where a fast-setting, low-shrink solution is needed: Spakfilla Rapid is a ready-mixed, lightweight filler that is fast-drying, virtually non-shrinking, non-cracking, and needs only one application. It's ideal for filling wallboard, wood, masonry, plaster, primed nail heads, and plasterboard.
For joints where movement occurs — skirting boards, architraves, cornice lines, window frames: Selleys No More Gaps is a durable, flexible acrylic gap sealant for interior and exterior applications where joint movement causes rigid fillers to crack or fall out.

The rule: If the two surfaces on either side of a gap can move independently of each other, you need a flexible product, not a rigid filler.

For a full breakdown of which product family applies to your specific repair scenario, see our guide on Wall Filler vs. Wood Filler vs. Gap Filler: Which Selleys Product Do You Actually Need?

Failure Mode 2: Insufficient Surface Preparation

The symptom: The filler appears to bond initially, then lifts cleanly away from the surface — sometimes in a single piece — days or weeks after application.

The cause: Filler cannot bond to dust, grease, loose paint, or moisture. These contaminants sit between the filler and the substrate, blocking mechanical adhesion. Repair performance comes down heavily to surface condition before you apply anything. If the surface isn't thoroughly cleaned of rust, dust, or old paint, the filler won't bond properly and the repair layer becomes prone to cracking and peeling.

Flaking paint around the repair zone is a particularly common oversight. Professional prep means cutting back to completely sound material — if there's flaking paint within 50mm of the repair area, remove it. It will lift eventually and take the new repair with it.

The fix: Before applying any Selleys filler:

Remove all loose, flaking, or crumbling material from the repair zone
Brush or vacuum out dust — don't simply wipe with a dry cloth
Degrease any surface that may have been touched by hand oils, cooking vapour, or cleaning products
Ensure surfaces are clean, dry, and free of loose material before pressing filler into place
Clean with Selleys Sugar Soap if contamination is suspected, then allow to dry fully before filling

Failure Mode 3: Overfilling in a Single Coat

The symptom: The repair sinks below the surface level as it dries, leaving a visible depression. Or the filler surface skins over quickly but cracks appear across it as the interior mass continues to shrink.

The cause: Water-based fillers cure by evaporation. When applied too thickly in a single pass, the outer surface skins and hardens while the inner mass is still wet. That mismatch in mechanical properties lets internal stresses build as the inner material shrinks — and the result is cracking that can propagate up through primer and topcoat.

For wood specifically, cracking usually comes from one of two things: filling a deep hole in a single pass (shrinkage) or the wood itself expanding and contracting too much for a rigid filler to handle.

The fix: Apply filler in layers, not in a single thick application.

For deeper cracks, apply in 6mm layers, allowing each layer to dry before applying the next.
Fill a deep gap in one pass and it often shrinks as it dries, leaving a sunken line. Build it up in two or three thin layers instead for a cleaner result.
For thin, near-flush repairs with Spakfilla Rapid, the low-shrink formulation means a single flush application delivers good results — don't overfill expecting shrinkage, because there's very little.

Failure Mode 4: Painting Before the Filler Has Fully Cured

The symptom: Paint bubbles, wrinkles, or cracks appear over the repair days or weeks after painting. The repair looks fine immediately after painting, then deteriorates.

The cause: Surface-dry doesn't mean fully dry. When paint is applied over incompletely cured filler, it seals moisture inside the repair mass. That moisture continues to evaporate, driving shrinkage and cracking from within — but now the cracking is transmitted through the paint film above it. Filler that hasn't fully cured will crack or shrink under a fresh coat.

Cold or humid conditions significantly extend cure times, particularly for applications more than 2mm deep.

The fix: Respect the cure window on the product data sheet, and adjust for conditions:

Selleys Product	Minimum Time to Sand	Minimum Time to Paint (Water-Based)	Minimum Time to Paint (Oil-Based)
Spakfilla Rapid	~30 min (thin fills)	Once fully set	24 hours
Spakfilla Heavy Duty	2–3 hours (2mm fills)	After sanding and dusting	After sanding and dusting
No More Gaps	~30 min (skinned)	~30 min (skinned)	~30 min (skinned)

Spakfilla Rapid can be sanded and painted with water-based paints once completely set — approximately 30 minutes in thin applications. For applications more than 2mm deep, or in cold or humid conditions, allow longer. For oil-based paints, wait 24 hours before painting.

For deep fills, three thin coats beats one thick coat. Each coat needs to dry completely (24 hours minimum for deep fills) to prevent shrinkage dips that show through paint.

Failure Mode 5: Using Rigid Filler Across a Movement Joint

The symptom: The filler cracks along the same line repeatedly, no matter how many times you re-fill it. The crack follows the junction between two different materials — wall and ceiling, skirting board and wall, door frame and plaster.

The cause: Buildings move. Thermal expansion and contraction, seasonal humidity changes, and structural settlement all cause surfaces to shift relative to each other — even by fractions of a millimetre. A rigid filler applied across a movement joint has zero ability to accommodate that shift.

Selleys No More Gaps Multipurpose is formulated specifically for this situation. Its water-based acrylic delivers flexibility and long-term resistance to cracking and crumbling, making it the right choice for sealing gaps and joints where expansion and contraction occurs — such as between a skirting board and a wall, where rigid fillers and less flexible gap fillers will crack and fall out. Rigid fillers simply can't handle that kind of movement.

The fix: Work out whether the crack sits within a surface (a substrate crack — use rigid filler) or between two surfaces (a movement joint — use flexible gap filler).

Selleys No More Gaps Multipurpose has greater than 300% elongation and up to ±15% movement capability. That means it stretches and recovers with the joint rather than cracking under stress.

Common movement joints that should always use flexible filler:

Between window and door frames and the wall, along skirting boards, around cornices, in corners or between walls and ceilings, between cabinets and cupboards, around air conditioning vents and the ceiling.

Also worth noting the product's limits: the maximum recommended gap size is 25mm wide by 12mm deep, and No More Gaps is not recommended where significant joint movement occurs, such as gaps between tongue and groove boards or weatherboard overlaps.

For a detailed walkthrough of the skirting board and architrave repair workflow — including when to use flexible vs. rigid filler on the same piece of trim — see our guide on How to Repair and Fill Skirting Boards, Architraves, and Timber Trim Before Painting.

Failure Mode 6: Poor Adhesion on Glossy or Sealed Surfaces

The symptom: The filler peels away from the edges of the repair, or the paint over the repair lifts in sheets, even though the filler appeared to bond initially.

The cause: Filler relies on mechanical adhesion — microscopic penetration into a porous surface. A glossy, sealed, or heavily painted surface has almost no porosity. The filler sits on top of the gloss layer rather than bonding into the substrate, and any stress — paint shrinkage, thermal movement, or even light impact — is enough to break that weak interface.

This is especially common on walls previously painted with high-sheen or semi-gloss finishes, timber trim coated with gloss enamel, and surfaces cleaned with wax-based products.

The fix: Two options, depending on the surface:

Mechanical abrasion: Sand the repair area with 80–120 grit sandpaper to break through the gloss layer and create a solid mechanical key. Always sand through to a mechanically sound layer — don't apply over loose, flaking, or glossy paint.
Priming: Apply a bonding primer over the sanded surface before filling, particularly on high-gloss enamel or varnished timber. Dust off any loose material before applying undercoat and painting.

On timber trim specifically, lightly sanding the gloss finish before applying Selleys Plastic Wood or No More Cracks will dramatically improve adhesion and eliminate the risk of edge lifting when the topcoat goes on.

Failure Mode 7: Skipping the Primer Coat Before Painting

The symptom: The paint over the repair looks patchy, has a different sheen to the surrounding wall, or the repair is visible as a 'witness mark' even after two coats of paint.

The cause: Bare, sanded filler is highly porous and absorbs the first coat of paint unevenly, creating a 'flashing' effect where the repair appears as a dull or shiny spot against the surrounding wall. This isn't a filler performance issue — it's a painting system issue. But it causes many DIYers to re-fill and re-sand unnecessarily when the only missing step was a primer coat.

The same principle that applies to fresh drywall applies here: without primer over a porous surface, paint soaks in and builds differently across the repair vs. the surrounding wall, creating shiny and dull spots that no amount of extra topcoats will fully fix.

The fix:

After sanding, clean dust and filler particles using Selleys Sugar Soap or Selleys Super Kleen.
Apply one coat of a water-based primer or sealer over the repair and allow to dry fully.
Lightly sand with 240 grit if the primer raises the surface.
Apply finish coats — two coats are typically needed to fully obscure a repair on older, oxidised walls.

For exterior repairs using Spakfilla Heavy Duty, the filler should be protected from rain until dry and painted over with 2 coats of paint.

Quick Diagnosis: What Your Filler Is Telling You

Failure Symptom	Most Likely Cause	Selleys Solution
Crack reappears along the same line	Movement joint — wrong product	Selleys No More Gaps Multipurpose
Filler sinks or dimples after drying	Applied too thick in one pass	Apply in 6mm layers; use Spakfilla Rapid for thin fills
Filler lifts cleanly from surface	Poor surface prep or glossy substrate	Sand, clean, prime; re-apply correct product
Paint bubbles or wrinkles over repair	Painted before full cure	Wait for full cure; adjust for cold/humid conditions
Filler cracks across the surface	Too thick, or wrong product for substrate	Layer correctly; match product to substrate
Repair visible as shiny/dull spot	No primer before painting	Apply bonding primer after sanding
Filler falls out of movement joint	Rigid filler used where flexibility needed	Switch to No More Gaps for all movement joints

How to Remove a Repair That Didn't Hold and Start Again

A repair that hasn't performed cannot simply be filled over. Remove the compromised material first, then do it right.

Score the edges of the existing repair with a utility knife to break the bond perimeter
Remove all loose material — use a scraper or putty knife to lift delaminated filler; leave no partially bonded material in place
Undercut the edges slightly if the repair is in a crack, so the fresh filler has a mechanical key to grip
Clean thoroughly — vacuum dust, wipe with a damp cloth, allow to dry completely
Assess the substrate — is the surface glossy, porous, or contaminated? Address it before re-filling
Select the correct product using the diagnosis table above
Apply in layers where depth exceeds 6mm, and observe full cure times before painting

Key Takeaways

Product selection is the most common cause of repeat repairs. Rigid fillers used at movement joints will always crack — unlike rigid fillers, flexible gap fillers retain flexibility once cured, which is exactly what movement joints need.
Surface preparation is non-negotiable. Filler applied over dust, grease, or gloss paint will not achieve lasting adhesion regardless of product quality.
Layer depth matters. For deeper cracks, apply in 6mm layers, allowing each layer to dry before the next. Single thick applications cause shrinkage cracking.
Cure time is not the same as dry time. A surface-dry repair can still contain trapped moisture that causes paint deterioration weeks later. Observe product-specific cure windows, especially in cold or humid conditions.
Primer is part of the repair system, not optional. Skipping primer over a sanded repair causes uneven paint absorption and visible witness marks — a problem that's often misdiagnosed as a product issue.

Conclusion

Every cracking, shrinking, or delaminating filler repair is a diagnostic opportunity. The seven failure modes described here — wrong product, inadequate prep, excessive single-coat depth, premature painting, rigid filler at movement joints, poor adhesion on glossy surfaces, and skipping primer — account for the vast majority of DIY repair outcomes that don't last. Each has a clear, preventable cause and a direct solution in the Selleys product range.

Wall and wood repair is a system, not a single step. Selleys Spakfilla Rapid and Spakfilla Heavy Duty are engineered for rigid substrates where no movement occurs. No More Gaps carries an estimated service life of 20 years when applied correctly to appropriate movement joints. But no product, however well formulated, will outperform the preparation and technique surrounding it.

If it's Selleys, it works. With the right product in the right place, prepared and applied correctly, you get results that hold — the first time and every time after.

For the complete workflow on specific repair scenarios, explore the related guides in this series:

How to Fill Cracks and Holes in Walls and Plasterboard: A Step-by-Step Selleys Guide — for interior wall repairs from hairline cracks to large plasterboard voids
How to Repair and Fill Skirting Boards, Architraves, and Timber Trim Before Painting — for the two-product approach to trim repair
How to Repair Rotted, Damaged, or Split Exterior Timber with Selleys Wood Fillers — for high-stakes exterior timber rebuilding

References

Selleys Australia. "Spakfilla Rapid Technical Data Sheet, Version 3.0." Selleys.com.au, 2020. https://www.selleys.com.au/media/5flebzp2/selleys-spakfilla-rapid-tds.pdf
Selleys Australia. "Spakfilla Heavy Duty Product Page." Selleys.com.au, 2024. https://www.selleys.com.au/products/putty-and-fillers/wall-filler/selleys-spakfilla-heavy-duty/
Selleys Australia. "No More Gaps Multipurpose Product Page." Selleys.com.au, 2024. https://www.selleys.com.au/products/putty-and-fillers/flexible-gap-fillers/selleys-no-more-gaps-multipurpose/
Selleys Singapore. "No More Gaps Technical Data Sheet." Selleys.com.sg, 2020. https://selleys.com.sg/wp-content/uploads/2020/05/TDS-No-More-Gaps.pdf
Queen of the Brush. "Why Do DIY Wall Repairs Crack?" QueenOfTheBrush.co.nz, 2024. https://queenofthebrush.co.nz/diy-wall-repairs/
SYBON Professional. "Why Body Filler Cracks — Causes, Repair Steps and Prevention." SuperSybon.com, January 2026. https://www.supersybon.com/car-body-filler-putty/why-body-filler-cracks
O'Reilly, B. "Achieve Maximum Adhesion Through Fail-Proof Prep." inPAINT Magazine, 2023. https://inpaintmag.com/achieve-maximum-adhesion-through-fail-proof-prep/
Smooth Tech Co. "Best Practices for Surface Prep Before Filling or Fairing." SmoothTechCo.com, August 2025. https://smoothtechco.com/blogs/news/best-practices-for-surface-prep-before-filling-or-fairing
ThePlywood.com. "Wood Filler Guide (2026): What Works, What Fails & What to Avoid." ThePlywood.com, April 2026. https://theplywood.com/wood-filler-guide/

Frequently Asked Questions

What is Selleys Spakfilla Heavy Duty: A multi-purpose, tough, durable, light grey ready-mixed filler

Is Spakfilla Heavy Duty suitable for exterior use: Yes

Is Spakfilla Heavy Duty suitable for interior use: Yes

What surfaces is Spakfilla Heavy Duty designed for: Plaster, plasterboard, brick, masonry, and primed metal

Does Spakfilla Heavy Duty suit surfaces with structural movement: No

What is Selleys Spakfilla Rapid: A ready-mixed, lightweight, fast-drying filler

Is Spakfilla Rapid virtually non-shrinking: Yes

Does Spakfilla Rapid require multiple applications: No, one application is sufficient

What surfaces is Spakfilla Rapid suitable for: Wallboard, wood, masonry, plaster, plasterboard, primed nail heads

How long before Spakfilla Rapid can be sanded (thin fills): Approximately 30 minutes

How long before Spakfilla Rapid can be painted with water-based paint: Once completely set

How long before Spakfilla Rapid can be painted with oil-based paint: 24 hours

Does cold weather affect Spakfilla Rapid cure time: Yes, longer drying time required

Does humidity affect Spakfilla Rapid cure time: Yes, longer drying time required

What is Selleys No More Gaps: A durable, flexible acrylic gap sealant

Is No More Gaps suitable for movement joints: Yes

Is No More Gaps suitable for interior use: Yes

Is No More Gaps suitable for exterior use: Yes

What is the elongation/stretch capability of No More Gaps Multipurpose: Greater than 300%

What is the movement capability of No More Gaps Multipurpose: Up to +/- 15%

What is the estimated service life of No More Gaps when correctly applied: 20 years

What is the maximum recommended gap width for No More Gaps: 25mm

What is the maximum recommended gap depth for No More Gaps: 12mm

Is No More Gaps recommended for tongue and groove board gaps: No

Is No More Gaps recommended for weatherboard overlap gaps: No

What is the most common cause of repeat filler repairs: Wrong product selection

Why does rigid filler crack at movement joints: It has no elasticity to accommodate surface movement

What type of product should be used at movement joints: Flexible gap filler, not rigid filler

Name a common movement joint requiring flexible filler: Between skirting board and wall

Is the junction between wall and ceiling a movement joint: Yes

Should rigid filler be used around window frames: No, use flexible filler

Should rigid filler be used around door frames: No, use flexible filler

Why does filler lift cleanly from a surface: Poor surface preparation or contaminated substrate

Can filler bond over dust: No

Can filler bond over grease: No

Can filler bond over loose paint: No

What should be used to degrease a surface before filling: Selleys Sugar Soap

How far back should flaking paint be removed before filling: Within 50mm of the repair area

What is the maximum single-coat application depth before layering is required: 6mm

Why does filler crack when applied too thickly in one pass: Outer surface skins while inner mass is still wet and shrinking

How should deep repairs be built up: In 6mm layers, allowing each layer to dry before the next

Does surface-dry mean fully cured: No

What happens when paint is applied over incompletely cured filler: Trapped moisture causes shrinkage and cracking under the paint film

Can filler bond to a glossy surface without preparation: No

What causes filler to peel from glossy surfaces: Lack of porosity prevents mechanical adhesion

What grit sandpaper should be used to key a glossy surface before filling: 80–120 grit

What is the alternative to sanding a glossy surface before filling: Apply a bonding primer

Why does a repaired area look patchy after painting: Bare sanded filler absorbs paint unevenly without primer

Is primer optional before painting over sanded filler: No, primer is required

What effect does skipping primer cause: Flashing — dull or shiny spots visible against surrounding wall

What grit sandpaper should be used after priming: 240 grit

How many finish coats are typically needed to obscure a repair on older walls: Two coats

What product should be used to clean dust after sanding: Selleys Sugar Soap or Selleys Super Kleen

How many coats of paint are required over exterior Spakfilla Heavy Duty repairs: Two coats

Should exterior Spakfilla Heavy Duty repairs be protected from rain before drying: Yes

What is the first step in removing a failed repair: Score the edges with a utility knife

Why should edges be undercut when re-filling a crack: To create a mechanical key for the fresh filler

What should never be left in place before re-filling: Partially bonded or delaminated filler material

What is the governing principle of all filler performance: Filler is only as good as the system it sits within

Does filler performance depend solely on product quality: No, preparation and technique are equally critical

What distinguishes a substrate crack from a movement joint crack: Substrate crack is within a surface; movement joint crack is between two surfaces

Which product is correct for a crack within a plaster wall: Rigid filler such as Spakfilla Heavy Duty

Which product is correct for a crack between wall and cornice: Flexible filler such as No More Gaps

Why does the same crack reappear repeatedly after rigid filling: The underlying joint continues to move

What causes paint to bubble over a repaired area weeks after painting: Moisture trapped under paint from incompletely cured filler

Does Spakfilla Heavy Duty require a cure time before painting: Yes, after sanding and dusting

What is the minimum sand time for Spakfilla Heavy Duty at 2mm fills: 2–3 hours

Is one thick coat of filler preferable to multiple thin coats: No, multiple thin coats are preferable

What product is recommended for primed nail head filling: Spakfilla Rapid

Can Spakfilla Rapid be used on wood: Yes

Can Spakfilla Heavy Duty be used on brick: Yes

Can Spakfilla Heavy Duty be used on masonry: Yes

What causes a witness mark after painting a repair: Skipping the primer coat before painting

Label Facts Summary

Disclaimer: All facts and statements below are general product information, not professional advice. Consult relevant experts for specific guidance.

Verified Label Facts

Selleys Spakfilla Heavy Duty

Product type: Multi-purpose, tough, durable, light grey ready-mixed filler
Suitable for interior use: Yes
Suitable for exterior use: Yes
Designed surfaces: Plaster, plasterboard, brick, masonry, and primed metal
Suitable for surfaces with structural movement: No
Minimum time to sand (2mm fills): 2–3 hours
Time to paint (water-based): After sanding and dusting
Time to paint (oil-based): After sanding and dusting
Exterior applications require protection from rain until dry: Yes
Exterior applications require 2 coats of paint over repair: Yes

Selleys Spakfilla Rapid

Product type: Ready-mixed, lightweight, fast-drying filler
Virtually non-shrinking: Yes
Number of applications required: One
Suitable surfaces: Wallboard, wood, masonry, plaster, plasterboard, primed nail heads
Minimum time to sand (thin fills): Approximately 30 minutes
Time to paint with water-based paint: Once completely set
Time to paint with oil-based paint: 24 hours
Cold weather affects cure time: Yes, longer drying time required
Humidity affects cure time: Yes, longer drying time required
Applications more than 2mm deep require longer drying times: Yes

Selleys No More Gaps Multipurpose

Product type: Durable, flexible acrylic gap sealant
Suitable for interior use: Yes
Suitable for exterior use: Yes
Suitable for movement joints: Yes
Elongation/stretch capability: Greater than 300%
Movement capability: Up to +/- 15%
Estimated service life when correctly applied: 20 years
Maximum recommended gap width: 25mm
Maximum recommended gap depth: 12mm
Recommended for tongue and groove board gaps: No
Recommended for weatherboard overlap gaps: No
Minimum time to skin (for painting): Approximately 30 minutes

General Product Claims

Filler is only as good as the system it sits within — substrate, preparation, product selection, application depth, cure time, and topcoat all interact
Rigid fillers used at movement joints will always crack due to inability to accommodate surface movement
Filler cannot bond to dust, grease, loose paint, or moisture
Flaking paint within 50mm of the repair area should be removed before filling
Maximum recommended single-coat application depth before layering is 6mm
Deep repairs should be built up in 6mm layers, allowing each layer to dry before the next
Surface-dry does not mean fully cured; trapped moisture can cause shrinkage and paint cracking weeks later
Paint applied over incompletely cured filler seals in moisture, causing shrinkage and cracking through the paint film
Glossy or sealed surfaces lack sufficient porosity for mechanical adhesion; filler sits on top rather than bonding into the substrate
80–120 grit sandpaper is recommended to key a glossy surface before filling
A bonding primer may be used as an alternative to sanding on high-gloss enamel or varnished timber
Bare, sanded filler is highly porous and absorbs paint unevenly without a primer coat, causing visible flashing (dull or shiny spots)
240 grit sandpaper is recommended after priming if the primer raises the surface
Two finish coats are typically required to fully obscure a repair on older, oxidised walls
Selleys Sugar Soap or Selleys Super Kleen is recommended for cleaning dust after sanding
A substrate crack (within a surface) requires rigid filler; a movement joint crack (between two surfaces) requires flexible filler
Common movement joints requiring flexible filler include: skirting board/wall junctions, wall/ceiling junctions, window frames, door frames, cornices, cabinet surrounds, and air conditioning vent surrounds
Failed repairs must be fully removed before re-filling; partially bonded material must not be left in place
Edges of cracks should be undercut slightly when re-filling to create a mechanical key for fresh filler
No product will outperform the preparation and technique surrounding it