A client had a self-levelling compound floor in his utility room that looked perfect when first laid. Two years later, sections were beginning to lift - thin flakes of SLC were peeling away from the screed below. When we examined the interface, the underside of the SLC had a fine, dusty layer. The primer had either not been applied or had been applied to a dusty screed surface and not bonded properly.
The SLC was structurally adequate - it just had not bonded. It had been sitting on top of the screed, not attached to it.
Why primer is necessary
Primer serves two functions:
1. Sealing porous substrates. Concrete and sand:cement screed are very porous. If SLC is poured onto an unprimed porous surface, the substrate rapidly absorbs the water from the SLC mix. This causes:
- The surface dries too fast, leaving the depth still wet and weak
- The mix stiffens before it has flowed and levelled properly
- The cured surface is sandy, weak and will crumble under light abrasion
2. Creating a bonding surface. Primer provides a chemical adhesion link between the substrate and the SLC. Without this link, the SLC may appear well-adhered initially but will delaminate under thermal cycling, vibration or moisture changes.
When can you skip the primer?
In practice: rarely. The only scenarios where primer may technically not be needed are:
- Applying SLC over an existing SLC surface that is still in primer-bonding condition
- Using a bonding-agent-based SLC system where primer is incorporated into the mix (some specialist products)
For all standard applications on concrete, screed, tiles, or wood substrates, always prime.
Choosing the right primer
Match the primer to the SLC product. Most SLC manufacturers specify which primer to use. The product data sheet will list compatible primers. When possible, use the primer from the same manufacturer as the SLC - compatibility is guaranteed.
| Substrate | Primer type |
|---|---|
| New concrete (porous) | Diluted primer first coat (1:3 to 1:5 with water), then full-strength second coat |
| Old concrete / screed | Standard SLC primer, full strength |
| Existing tiles (ceramic, porcelain) | SLC-specific tile primer or bonding agent |
| Wood subfloor | Flexible primer suitable for wood substrates |
| Anhydrite screed | Specific anhydrite-compatible primer (standard cement primers may not work) |
| Metal | Specialist bonding primer |
Anhydrite screed note: This is a common problem. Standard SLC primers are designed for cementitious substrates. Anhydrite (calcium sulphate) screed is not cementitious and standard cement-based SLC will not bond to it without a specific primer. Check the substrate before specifying.
How to apply primer correctly
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Clean and vacuum the floor. Dust, grease or loose material prevents the primer penetrating and bonding. Vacuum thoroughly - this is more important than people realise.
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Test for moisture. If the substrate is above the moisture threshold for your SLC product, the primer may not bond correctly. Test with a calibrated RH probe before priming.
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Apply primer to the whole surface. Use a brush, roller or broom to apply the primer generously and evenly. Work it into pores on rough surfaces.
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Two coats on very porous surfaces. On brand-new concrete or rough-faced screed, a diluted first coat (follow manufacturer's ratio) penetrates deeper before sealing. Allow to dry, then apply a second coat at full strength.
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Pour SLC when primer is tack-dry. The ideal state is slightly tacky - touch the surface with your knuckle and it should feel resistant and slightly sticky, not wet. If it is fully dry and more than a few hours have passed, re-prime.
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Do not let the primer dry out completely before pouring. A fully dried-out primer loses its tack and adhesion properties. If you cannot pour the SLC the same day as priming, re-prime the next day.
My tips on priming for SLC
Always vacuum before priming, not just sweep. A broom moves dust around. Dust contamination is the most common reason for primer failure on screed. Vacuum thoroughly and then prime within an hour.
Keep a small amount of primer for touch-ups. If you accidentally scuff the primed surface before pouring, you need to re-prime the disturbed area. Keep 10% of the primer back for this.
Check the temperature. Most primers should not be applied below 5°C or above 30°C. On cold ground floors in winter, the primer may not properly cure even if the air temperature is adequate.
Do not thin the primer more than specified. Diluting primer beyond the product recommendation reduces its sealing effectiveness. If you have a very porous surface that seems to soak up even diluted primer, apply a second coat rather than diluting further.
Use the Self-Levelling Compound Calculator to estimate how many bags you need before starting.