Choosing the right floor levelling compound starts with the subfloorānot the flooring being installed over it. Concrete, plywood, anhydrite screed, old adhesive residues and waterproof surface membranes all require different preparation systems.
The terms levelling compound, smoothing compound, latex and screed are often used interchangeably, but the products do not all perform the same job. Some are designed for rigid mineral subfloors, some include fibres to accommodate suitable timber substrates, and others are intended specifically for refurbishment work over old adhesive residues.
This guide compares the F. Ball Stopgap levelling compounds, repair products, primers and damp-proof membrane stocked by Wiltshire Wood Flooring Supplies. It will help you choose a complete preparation system before installing wood flooring or another compatible floor covering.
Choose Stopgap 300 HD for a strong general-purpose and heavy-duty smoothing compound over suitable absorbent and non-absorbent subfloors.
Choose Stopgap Fast-Track 30 when installation speed is the main priority.
Choose Stopgap 700 Superflex for suitable plywood, timber, steel and other substrates where a flexible fibre-reinforced compound is required.
Choose Stopgap 1200 Pro for refurbishment projects over suitable well-bonded adhesive residues, including bitumen and carpet-tile tackifier residues.
Use Stopgap Micro Rapid for feather edging and minor imperfections, or Stopgap 400 Repair for deeper local repairs.
Floor Levelling Compounds at a Glance
| Product | Best suited to | Application depth | Approximate coverage | Key advantage |
|---|---|---|---|---|
| Stopgap 300 HD | Sound absorbent and non-absorbent mineral subfloors; demanding domestic and commercial work | 2ā20mm | Approximately 5mĀ² at 3mm per 25kg bag | High-strength heavy-duty finish |
| Fast-Track 30 | Time-sensitive work over suitable absorbent and non-absorbent subfloors | 2ā10mm | Approximately 1.6kg/mĀ² per millimetre | Walkable in around 30 minutes and some coverings from around 45 minutes |
| Stopgap 700 Superflex | Plywood, suitable timber, steel, sand/cement and demanding flexible substrates | 2ā10mm | Approximately 5mĀ² at 3mm per 20kg bag | Flexible fibre-reinforced formulation |
| Stopgap 1200 Pro | Refurbishment projects and suitable well-bonded adhesive residues | 2ā10mm unfilled; up to 30mm with approved aggregate | Approximately 1.98kg/mĀ² per millimetre | Often applied over suitable residues without priming |
| Micro Rapid | Feather edging, screw heads, plywood joints and minor surface defects | Feather edge to approximately 4mm | Depends on repair area | Very fast local finishing and repair |
| Stopgap 400 Repair | Holes, cracks, damaged screed, steps, ramps and deeper local repairs | 2ā50mm, subject to the specified mix | Depends on repair volume | Slump-free deep repair mortar |
Stopgap 300 HD
A strong smoothing compound for demanding installations
Stopgap 300 HD is a fast-drying, heavy-duty self-smoothing compound designed for sound absorbent and non-absorbent internal subfloors.
It is the strongest general-purpose option in the current WWFS levelling range. Its high compressive strength and dimensional stability make it suitable for domestic, commercial and industrial environments where the prepared floor needs to withstand demanding use.
Suitable prepared subfloors include:
- Concrete.
- Sand and cement screeds.
- Prepared calcium sulphate or anhydrite screeds.
- Sound flooring-grade asphalt.
- Granolithic and terrazzo floors.
- Prepared ceramic and quarry tiles.
- Suitable epoxy and polyurethane resin surfaces.
- Compatible waterproof surface membranes.
- Minimal, firmly bonded adhesive residues where approved.
Why choose Stopgap 300 HD?
- Application depth from 2mm to 20mm.
- Approximately 20ā30 minutesā working time.
- Walkable after approximately 90 minutes.
- Ready for compatible floor coverings from around six hours on absorbent substrates.
- Suitable for hand or pump application.
- High strength for light to heavy-duty environments.
- Approximately 5mĀ² coverage at 3mm per 25kg bag.
Choose 300 HD where strength and durability matter more than achieving the very shortest possible drying time.
View Stopgap 300 HDStopgap Fast-Track 30
Rapid setting and rapid drying preparation
Stopgap Fast-Track 30 is designed for projects where the floor needs to return to use as quickly as possible.
It is suitable for light to heavy-duty work over correctly prepared absorbent and non-absorbent subfloors. Under suitable conditions, it can be walked on after approximately 30 minutes, with certain textile and resilient floor coverings installed from around 45 minutes.
Why choose Fast-Track 30?
- Application depth from 2mm to 10mm.
- Approximately 15ā20 minutesā working time.
- Walkable from around 30 minutes.
- Suitable coverings from around 45 minutes under the stated conditions.
- Suitable for hand or pump application.
- Ideal for refurbishment projects with limited downtime.
- Suitable for domestic and commercial work.
Stopgap 700 Superflex
Fibre reinforcement for more demanding substrates
Stopgap 700 Superflex is a fast-drying fibre-reinforced smoothing compound developed for plywood, suitable timber, steel and prepared mineral subfloors.
Timber-based floors can experience small amounts of movement that are not normally present in a rigid concrete slab. The fibre-reinforced formulation gives Stopgap 700 greater flexibility than a conventional general-purpose smoothing compound.
Suitable prepared subfloors include:
- Flooring-grade plywood.
- Suitable stable timber substrates.
- Sand and cement screeds.
- Prepared concrete.
- Steel subfloors.
- Suitable raised-access systems.
Why choose Stopgap 700?
- Flexible fibre-reinforced formulation.
- Application depth from 2mm to 10mm.
- Approximately 20ā30 minutesā working time.
- Walkable after around 60ā90 minutes, depending on conditions.
- Suitable floor coverings from around three hours at 3mm.
- Mixes with clean water.
- Approximately 5mĀ² coverage at 3mm per 20kg bag.
Stopgap 1200 Pro
A two-part smoothing compound for difficult existing floors
Stopgap 1200 Pro is supplied as a powder with its matching liquid component. It is particularly useful during refurbishment work because it can often be applied directly over suitable, well-bonded adhesive residues without first applying a primer.
These can include certain bitumen adhesive residues and carpet-tile tackifiers, provided the residue is thin, firmly bonded, uncontaminated and accepted within the current F. Ball specification.
Why choose Stopgap 1200 Pro?
- Ideal for many refurbishment projects.
- Suitable over approved well-bonded adhesive residues.
- Often avoids the need to prime suitable residues.
- Application depth from 2mm to 10mm unfilled.
- Can be extended to approximately 30mm using approved aggregate.
- Walkable after approximately two hours.
- Ready for many compatible floor coverings from around four hours.
- Low-odour two-component formulation.
Repairing the Floor Before Levelling
A full smoothing compound should not be expected to repair every crack, hole, broken edge or deep area. Local defects should be repaired before the wider floor is levelled.
Stopgap Micro Rapid
Use for feather edging, minor surface imperfections, plywood joints, screw heads and shallow local finishing work.
It can be applied from a feather edge to approximately 4mm and may be ready to cover within around 20ā30 minutes.
Stopgap 400 Repair
Use for deeper holes, damaged screeds, steps, ramps, cracks and substantial local repairs.
Its slump-free consistency allows horizontal, angled and certain vertical repairs from approximately 2mm to 50mm, subject to the required mix and aggregate.
Which Stopgap Primer Should I Use?
Priming improves adhesion, controls absorption and reduces the risk of the smoothing compound drying too quickly or developing pinholes.
The primer must be chosen according to the subfloor. P121, P131 and P141 are not interchangeable versions of the same product.
| Primer | Primary use | Typical substrates | Important distinction |
|---|---|---|---|
| Stopgap P121 | Calcium sulphate and anhydrite screeds | Correctly prepared, dry calcium sulphate screeds | Specialist acrylic primer for anhydrite rather than a general-purpose primer |
| Stopgap P131 | General absorbent and approved non-absorbent subfloors | Concrete, sand/cement, plywood, timber, terrazzo, steel and other specified surfaces | Versatile neoprene primer used at the dilution specified for the substrate |
| Stopgap P141 | Non-absorbent surfaces and waterproof membranes | Stopgap F77, resin surfaces and other approved non-absorbent substrates | Dries to a hard textured surface that creates a mechanical key |
Stopgap P121
Stopgap P121 is the primer to consider for correctly prepared calcium sulphate or anhydrite screeds. It controls absorbency and helps prevent smoothing compounds and adhesives from drying too quickly.
Calcium sulphate screeds require specialist moisture testing and preparation. Surface laitance must be removed and the screed must be dry to the required level before priming.
Stopgap P131
Stopgap P131 is the most versatile primer in the WWFS Stopgap range. It is used over many absorbent and approved non-absorbent substrates, with its dilution adjusted according to the surface.
On absorbent surfaces, the correct diluted coat helps control rapid water loss, pinholing and flow. On certain non-absorbent surfaces, it may be used neat where the current product specification permits.
Stopgap P141
Stopgap P141 is designed primarily for non-absorbent substrates. It contains recycled uPVC particles and dries to a hard, textured surface that improves the mechanical bond of the smoothing compound.
It is particularly important when applying most Stopgap smoothing compounds over a cured Stopgap waterproof surface membrane such as F77.
When Is a Damp-Proof Membrane Needed?
Moisture must be tested before levelling or installing wood flooring. A floor can appear dry at the surface while still containing excessive residual construction moisture.
Stopgap F77 is a two-part epoxy waterproof surface membrane used over suitable cementitious subfloors to impede residual construction moisture and rising damp.
Within its approved specification, F77 can provide moisture protection up to 98% relative humidity. On suitable underfloor-heated screeds, F. Ball states a lower maximum of 90% relative humidity and a specified application rate.
Typical F77 and smoothing-compound system
Stopgap 1200 Pro is a useful exception. It can be applied directly over fully cured F77 within the specified direct-application window, avoiding the P141 stage where all current F. Ball conditions are satisfied.
Which Compound Should I Use for My Subfloor?
| Subfloor or situation | Product to consider | Primer or preparation |
|---|---|---|
| Dry concrete or sand/cement screed | 300 HD or Fast-Track 30 | P131 at the dilution specified for the substrate |
| Plywood or suitable stable timber | 700 Superflex | P131 where specified; sheets must be correctly graded and securely fixed |
| Calcium sulphate or anhydrite screed | Compatible Stopgap smoothing compound selected for the project | Mechanical preparation followed by P121 |
| Suitable old adhesive residues | 1200 Pro | Often no primer over approved residues; inspect and prepare carefully |
| Non-absorbent tile or resin surface | 300 HD, Fast-Track 30 or another compatible compound | P141 or the specific primer stated for the selected product and substrate |
| Cured F77 waterproof membrane | Compatible Stopgap compound | Normally neat P141; 1200 Pro may be applied directly within its specified window |
| Minor lips, screw heads or shallow imperfections | Micro Rapid | Prime where required by the substrate |
| Deep holes or damaged screed | 400 Repair | Prepare and prime in accordance with the repair specification |
How Much Levelling Compound Do I Need?
The amount required depends on the floor area, average depth and product consumption.
A floor described as requiring 3mm of compound will not necessarily use exactly 3mm everywhere. High points, dips and variations mean the actual average thickness may be greater.
As a basic example, Stopgap 300 HD covers approximately 5mĀ² at 3mm from one 25kg bag. A nominal 30mĀ² floor at a genuine average depth of 3mm would therefore require approximately six bags before allowing for waste or local depth variation.
Allow additional material where:
- The subfloor has significant dips.
- A minimum finished height must be achieved.
- The floor runs into several rooms.
- There are complex edges, doorways or fixed units.
- The installation is far from replacement stock.
- The compound will be pumped or transferred over distance.
Preparing the Subfloor
Levelling compound must be applied to a correctly prepared, structurally sound surface.
- Identify the exact subfloor construction.
- Test and record moisture where relevant.
- Remove weak laitance, dust, paint, grease and contamination.
- Check that old adhesive residues are suitable to remain.
- Repair cracks, holes and damaged edges.
- Confirm the floor is sufficiently stable.
- Select and apply the correct primer.
- Fit barrier foam where the compound could escape beneath walls, units or openings.
- Set out mixing water, bags, tools and the application route.
Stopgap Barrier Foam helps contain smoothing compound around the perimeter and prevent it running into gaps or adjoining areas.
Mixing and Applying Levelling Compound
- Use clean buckets, clean water and clean mixing equipment.
- Measure the exact quantity of water stated for the product.
- Add the powder progressively while mixing at the recommended speed.
- Mix until the material is smooth and lump-free.
- Do not add extra water to improve flow.
- Pour immediately and spread to the required thickness.
- Maintain a wet edge between mixes.
- Use the correct smoothing trowel or gauge rake.
- Use a spiked roller only where appropriate and while the material remains workable.
- Protect the floor from draughts, direct heat and contamination while curing.
Common Levelling-Compound Mistakes
Calling every product ālatexā
Some products mix with water, while Stopgap 1200 Pro is supplied with a separate liquid component. Choose by technical suitability rather than the general term used on site.
Using a rigid compound over moving timber
Suitable plywood and timber substrates normally require a fibre-reinforced product such as Stopgap 700 rather than a standard rigid mineral smoothing compound.
Skipping the primer
Uncontrolled absorption can remove water too quickly, causing pinholes, poor flow and reduced bond. Non-absorbent surfaces may also require a textured bonding primer.
Using the wrong primer
P121 is primarily for calcium sulphate screeds, P131 is the versatile general primer, and P141 is designed to key non-absorbent surfaces. They should not be selected by pack colour or availability.
Levelling over moisture without testing
Smoothing compound does not stop moisture. Test the subfloor and install an approved DPM system where required.
Expecting compound to repair structural movement
Loose plywood, flexing boards, active cracks and weak screed must be repaired or stabilised before levelling.
Stretching a mix with extra water
Overwatering changes the designed water-to-powder ratio and may compromise the floor.
Mixing alone on a large area
Levelling compound has a limited working time. Larger areas may require one person mixing continuously while others pour and spread.
Installing wood flooring too soon
āReady to coverā depends on product, thickness, substrate, temperature and the floor covering being installed. Wood flooring and reactive adhesives may require different conditions from textile or resilient flooring.
Products for a Complete WWFS Subfloor System
Frequently Asked Questions
Which levelling compound is best for concrete?
Stopgap 300 HD is the strongest general-purpose choice for a sound prepared concrete floor. Choose Fast-Track 30 where drying speed is the priority.
Which levelling compound should I use over plywood?
Stopgap 700 Superflex is specifically designed for suitable plywood and other demanding substrates. The plywood must still be flooring grade, correctly fixed and adequately supported.
Which compound can go over old adhesive residue?
Stopgap 1200 Pro is designed for suitable well-bonded residues, including certain bitumen and tackifier residues, often without priming. Loose or unsuitable residues must still be removed.
What is the fastest Stopgap levelling compound?
Fast-Track 30 is the rapid option in the current WWFS range. It can be walked on after around 30 minutes and accepts certain floor coverings from around 45 minutes under suitable conditions.
Can I apply levelling compound without primer?
Some specific products and substrates allow this, notably Stopgap 1200 Pro over approved residues. Most systems require the primer stated for the substrate and compound.
Which primer should I use on concrete?
Stopgap P131 is normally the primer to consider on suitable absorbent concrete, diluted in accordance with the current F. Ball specification.
Which primer should I use on anhydrite screed?
Stopgap P121 is designed for correctly prepared, sufficiently dry calcium sulphate and anhydrite screeds.
Which primer should I use over F77?
Most Stopgap smoothing compounds require neat P141 over cured F77. Stopgap 1200 Pro may be applied directly within the specified F. Ball application window.
Can levelling compound stop damp?
No. Levelling compound does not replace a damp-proof membrane. Test moisture and install an approved product such as Stopgap F77 where the full specification requires it.
Can F77 be used over underfloor heating?
F. Ball permits F77 over suitable cementitious underfloor-heated screeds up to its stated moisture limit and application rate. The system must be tested and specified correctly.
Can I use F77 on anhydrite screed?
No. F. Ball waterproof surface membranes are not intended for use over calcium sulphate or anhydrite screeds.
What should I use for deep holes?
Use Stopgap 400 Repair for deeper local repairs. Micro Rapid is intended for shallow finishing, minor imperfections and feather edging.
Can levelling compound be applied to a feather edge?
Full smoothing compounds generally have a stated minimum thickness. Use Stopgap Micro Rapid for small feather-edged areas and finishing repairs.
How thick should levelling compound be under wood flooring?
It should be thick enough to achieve the required flatness while remaining within the productās permitted application range. A nominal 3mm coat is common, but the actual depth depends on the surveyed floor.
Can I glue wood flooring directly to levelling compound?
Yes, once the selected compound is fully dry and compatible with the wood flooring adhesive. Follow both manufacturersā preparation, moisture and drying requirements.
Our Final Recommendation
Choose the compound from the subfloor upwards
Start by identifying the subfloor, testing moisture and checking stability. Repair deep damage before levelling and choose the primer specifically required for the surface.
Use Stopgap 300 HD for strong general-purpose preparation, Fast-Track 30 when speed is critical, Stopgap 700 for suitable plywood and timber, and Stopgap 1200 Pro for approved adhesive residues and refurbishment work.
Use P121 on prepared calcium sulphate screeds, P131 as the versatile general primer and P141 to key suitable non-absorbent surfaces and waterproof membranes.
Where moisture is above the flooring systemās permitted level, specify Stopgap F77 only after testing the subfloor and confirming the complete preparation and flooring build-up.
Shop Subfloor Preparation Shop Levelling PrimersProduct suitability, primer dilution, application depth, coverage and drying depend on the substrate, moisture, temperature, humidity and installed floor covering. Always consult the latest F. Ball Technical Data Sheet and subfloor preparation guidance before use. This guide supports product selection but does not replace moisture testing, structural assessment or a project-specific flooring specification.