Most people searching for how to seal a concrete floor have one of two problems. Either the floor is bare and keeps making dust, or a new floor covering is about to go down and someone has said the screed "needs sealing". Those are different jobs, and the product that fixes the first can cause trouble with the second.
Short answer: should you seal a screed floor?
Seal a concrete or screed floor only if it will stay exposed. A dust proofing sealer or densifier binds a bare surface so it stops dusting and is easier to clean. If a floor finish is going on top, do not seal it. Remove laitance and contamination mechanically, then apply the primer named by the maker of the levelling compound or adhesive.
The table below sums it up.
| Your situation | What to use | What to avoid |
|---|---|---|
| Garage, workshop, store or plant room left bare | Dust proofing sealer or silicate densifier on a sound, clean surface | Sealing over loose dust or a crumbling top |
| Screed about to receive vinyl, LVT, carpet or wood | Mechanical preparation, then the specified primer and smoothing compound | Any sealer, PVA or floor paint not named by the flooring system |
| Screed about to be tiled | The tile adhesive maker's primer, after laitance is removed | Sealers that leave a film the adhesive cannot key into |
| Calcium sulphate (anhydrite) screed | Laitance sanded off, then a compatible primer | Leaving the laitance on, or using a cement product without the right primer |
| Surface that crumbles, scratches deeply or sounds hollow | Grinding back or repair first | Any sealer: it cannot hold a weak surface together |
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Why concrete and screed floors dust
A concrete or sand and cement floor that keeps producing grey powder usually has a weak top layer. The common causes are:
- Laitance. A thin, weak, dusty skin forms on concrete and some screeds as they cure. It is the main reason a new floor dusts, and the main reason adhesives and compounds fail to bond.
- Poor curing. If the surface dries too quickly, the top never gains full strength. The Mineral Products Association's screed data sheet says ready to use levelling screeds should be covered with plastic sheeting or similar to retain moisture for at least five days.
- A weak mix or poor compaction. A screed that was too lean or not compacted properly can break down under foot. That is a defect in the screed, not a surface problem.
- Wear. Traffic, trolleys and tyres slowly abrade a bare surface, especially one that was never meant to be walked on as a finish.
That last point matters more than most people realise. BS 8204-1, the code of practice for concrete bases and levelling screeds, as summarised by the MPA, defines a levelling screed as one "suitably finished to obtain a defined level and to receive the final flooring". A wearing screed is a different thing: one "that serves as a flooring". Most domestic screeds are levelling screeds. They are made to be covered, so if one is left bare it will often dust, and a sealer is a sensible way to manage that rather than a cure for the underlying design.
Sealers for exposed floors: what each type does
For a floor that will stay bare, there are three common families of product. The figures below come from Sika's UK product pages; other makers publish similar data, and you should always follow the sheet for the product in your hand.
| Type | Example | How it works | Coverage per coat | Notes |
|---|---|---|---|---|
| Sodium silicate densifier | Sikafloor CureHard-24 | Penetrates and reacts chemically to densify the surface | 0.15 to 0.25 litres per m² | Scrubbed in for at least 30 minutes, then the excess rinsed off. A second coat may be needed on porous or rough floors. Sika says the maximum hardening effect comes after 7 days at the earliest. Colourless and solvent free |
| Water based acrylic | Sikafloor ProSeal W | Forms a thin acrylic seal; also used as a curing compound | 0.1 to 0.2 litres per m² | Sold as an anti dust treatment for existing floors. Each coat must be tack free before the next, roughly 40 to 90 minutes depending on temperature |
| Solvent based acrylic | Sikafloor ProSeal-12 | Acrylic resin solution that cures, hardens and seals | 0.10 to 0.25 litres per m² | Gives a non dusting, wear resistant surface. Once cured it can only be removed mechanically. Tools are cleaned with solvent thinner |
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Floor paints and resin coatings sit beyond these. They build a thicker coloured film and need a well prepared, usually ground or shot blasted, surface. We do not lay resin coatings, but we do prepare floors for them.
How to seal a bare concrete floor
- Check the surface is sound. Scrape it firmly with a key or screwdriver. If it scores deeply, crumbles or keeps producing fresh dust after sweeping, a sealer will not fix it. See the repair section below.
- Remove what will stop the sealer soaking in. Oil, paint, old sealer and heavy laitance need taking off mechanically.
- Clean thoroughly. Vacuum rather than sweep, so the fine dust is removed and not just moved about.
- Check porosity. Drop a little water on the floor. If it soaks in, a penetrating sealer will too. If it beads on the surface, the floor is closed or already sealed, and a penetrating product will sit on top.
- Apply to the data sheet. Use the stated coverage, do not flood the floor, and respect the time between coats.
- Leave it to harden. A silicate densifier keeps hardening for at least a week, so keep heavy traffic off where the sheet says so.
On a new slab, both Sika acrylic products are designed to go on as a curing compound and sealer in one step, which deals with dusting from the start.
Under flooring: prime, do not seal
Where a floor finish is going down, the surface has a different job. It has to let the primer, smoothing compound or adhesive soak in and grip. A sealer is designed to do the opposite: close the surface and stop things getting in.
That is why flooring makers ask for an open, clean surface, not a sealed one. Karndean's glue down installation guide asks for a subfloor that is "firm, hard, flat, level, smooth, dry, clean and dust-free". For concrete floors it says to mechanically abrade the surface to remove laitance, paint, plaster and adhesive, then apply a damp proof membrane where needed, a primer and an underlayment system to the product makers' instructions. Painted floors must have all paint mechanically removed. On power floated concrete, it recommends a water drop test for porosity: where water beads, grinding, scarifying or light shot blasting may be needed so the adhesive can soak in.
A sealed surface behaves like that closed, power floated one. The adhesive sits on the film instead of keying into the floor, so the bond is only as good as the bond of the sealer, and that is not what the adhesive maker tested. If the floor has already been sealed or painted, expect to have it ground off before the new floor goes down.
Primer is not the same as sealer
Primers for floors are designed to be part of the flooring system. They control suction so a smoothing compound does not dry too fast, bind any remaining surface dust and help the compound bond. They are matched to the base:
| Primer | Base | Mix or rate | Drying time |
|---|---|---|---|
| F. Ball Stopgap P131 | Absorbent: concrete, sand and cement screed | Applied diluted, F. Ball quotes about 100 m² coverage | 20 to 30 minutes at 20°C |
| F. Ball Stopgap P131 | Non absorbent: power floated concrete, tiles, terrazzo | Applied neat, F. Ball quotes about 50 m² coverage | About 60 minutes at 20°C |
| ARDEX P 51 | Highly absorbent cement screeds | 1 part primer to 5 parts water | See data sheet |
| ARDEX P 51 | Absorbent, smooth calcium sulphate screeds | 1 to 3 | See data sheet |
| ARDEX P 51 | Power floated concrete | 1 to 2 | See data sheet |
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Some compounds need no primer at all on certain bases. F. Ball says Stopgap 1200 can go directly over sound adhesive residues without priming. That is why the rule is simple: use the primer the compound or adhesive maker names, at the dilution they give, and nothing else.
Two other points before flooring goes down. Calcium sulphate screeds must have their surface laitance removed, and Karndean tells installers to ask the screed maker which primers and compounds suit the system. And the base must be dry: Karndean, for example, asks for less than 75% RH by hygrometer. Our guide to screeding before flooring covers moisture testing and flatness in detail.
When a dusty floor needs grinding or repair first
A sealer only works on a surface that is already sound. If the top is weak, the sealer soaks into a layer that will still break away under traffic, taking the sealer with it.
| What you see | Likely cause | What usually fixes it |
|---|---|---|
| Light dust, surface hard when scratched | Laitance or a dry, unsealed surface | Clean, then seal (bare floor) or grind and prime (covered floor) |
| Old sealer, paint or adhesive present | Previous treatment | Diamond grinding back to clean concrete |
| Dust keeps coming back, aggregate showing | Worn or weak top layer | Grind back and resurface with a bonded screed or compound |
| Crumbles under a key, breaks up under foot | Weak mix or poor compaction | Remove the weak screed and relay |
| Cracks, hollow areas, broken joint edges | Movement or loss of bond | Concrete floor repair or screed repairs |
Swipe the table sideways to see every column.
Grinding cuts away the top of a slab or screed in thin layers, taking off laitance, high spots, adhesive, floor paint and thin coatings. It cannot level a floor that slopes or dips over a large area, and grinding too deep can leave a screed thinner than its minimum depth, so it needs judgement. Our floor preparation page compares grinding with shot blasting and scarifying.
The dust from that work is not ordinary dust. Concrete and screed contain silica, and grinding produces respirable crystalline silica. The HSE says over 500 construction workers are believed to die from exposure to silica dust every year, and publishes separate guidance on controlling it with on-tool extraction. We only grind with dust extraction. If you are tempted to hire a grinder for a DIY job, read the HSE guidance first. Wet cement is also highly alkaline and the HSE warns it can cause serious burns if trapped against the skin, which is worth knowing if you are patching a floor yourself.
If your floor is a garage that you are thinking of screeding rather than sealing, our garage floor screed guide explains the options.
Get a fixed price for your floor
If you are not sure whether your floor needs sealing, grinding or repairing, send us a few photos of the surface and tell us:
- whether the floor will stay bare or get a floor finish, and which one;
- the area in m²;
- whether it is concrete, sand and cement screed or a flowing screed, if you know;
- what you have noticed: dust, cracks, hollow areas, old paint or adhesive;
- the postcode.
We will tell you what the floor actually needs and price the preparation or repair. You get a same day call back and a written, fixed price within 24 hours, agreed before any work starts. Contact us here or call 07466 650936.
Sources
- Data Sheet 22: Screeds (Issue 3, October 2023), Mineral Products Association, accessed October 2026
- BS 8204-1 Screeds, bases and in situ floorings: concrete bases and cementitious levelling screeds, BSI, accessed October 2026
- Sikafloor CureHard-24, Sika UK, accessed October 2026
- Sikafloor ProSeal W, Sika UK, accessed October 2026
- Sikafloor ProSeal-12, Sika UK, accessed October 2026
- Karndean Designflooring glue down installation guidelines (UK), Karndean, accessed October 2026
- Stopgap P131 general purpose floor primer, F. Ball and Co., accessed October 2026
- Stopgap 1200 high performance smoothing compound, F. Ball and Co., accessed October 2026
- ARDEX P 51 primer, ARDEX UK, accessed October 2026
- Construction dust, Health and Safety Executive, accessed October 2026
- Cement, Health and Safety Executive, accessed October 2026
