Priming a concrete floor before applying self-levelling compound (SLC) is one of the most important steps in the whole process — skip it and you risk the compound drying too fast, delaminating, or developing pinholes and cracks. Done correctly, priming seals the porous concrete surface, controls moisture absorption, and gives the SLC something to bond to. Here's how to do it right.
Why Priming Matters
Concrete is porous. Without a primer, it will rapidly suck moisture out of the self-levelling compound before it has time to flow and cure properly. This causes premature drying, weak adhesion, and surface defects. A good primer also helps reveal any remaining dust or contamination that could compromise the bond. Manufacturers such as Ardex and Mapei both specify priming as a mandatory step — always follow the SLC manufacturer's instructions, as using the wrong primer can void product warranties.
Step 1 — Prepare the Concrete Surface
No primer will fix a poorly prepared slab. Before you open a tin, the concrete must be:
- Clean and dry — sweep thoroughly, then vacuum up all dust and debris.
- Free of contaminants — remove any oil, grease, paint, adhesive residue, or curing compounds. These prevent bonding. Use a degreaser or mechanical grinding where needed.
- Structurally sound — tap the slab with a hammer to check for hollow or delaminating areas. Repair any cracks, spalls, or holes with a suitable patching compound and allow it to cure fully before priming.
- At the right moisture level — this is critical in New Zealand's varied climate. Excess moisture in the slab can cause SLC failure. A moisture vapour emission rate (MVER) test or plastic sheet test (tape a sheet of plastic to the floor for 24–48 hours and check for condensation) is a good starting point. If in doubt, consult a flooring professional or refer to BRANZ guidance on moisture in concrete floors.
Step 2 — Choose the Right Primer
Always use the primer recommended by your SLC manufacturer. Mixing brands can cause compatibility issues. Common primer types include:
| Primer Type | Best For | Notes |
|---|---|---|
| Water-based acrylic primer | Standard porous concrete | Most common for residential use |
| Epoxy primer | Dense, low-porosity or contaminated slabs | Two-part mix, longer working time |
| Diluted SLC primer | Highly porous or dusty slabs | Sometimes applied in two coats |
Products from Ardex, Mapei, and Sika are all widely available through NZ trade suppliers and stores like Bunnings, Mitre 10, and PlaceMakers.
Step 3 — Mix the Primer Correctly
Many primers are supplied as a concentrate and need to be diluted with water. The dilution ratio depends on the porosity of your slab:
- High porosity (older, rough, or dusty concrete) — use a more diluted mix for the first coat so it penetrates deeply.
- Low porosity (smooth, dense, or power-floated concrete) — use a stronger concentration or a specialist primer designed for dense substrates.
Always read the technical data sheet (TDS) for the exact ratio. Guessing the dilution is a common DIY mistake.
Step 4 — Apply the Primer
- Pour the primer onto the floor and spread it evenly using a short-pile roller or a wide brush for edges and corners.
- Work in manageable sections so the primer doesn't dry before you've spread it evenly.
- On very porous slabs, apply a second coat once the first is touch-dry — this is often recommended on older NZ concrete floors that have never been sealed.
- Make sure the primer reaches all edges, including along walls and around any penetrations.
- Do not puddle — excess primer pooling in low spots can actually weaken the bond. Spread it out.
Step 5 — Allow Correct Drying Time
This is where patience pays off. The primer must be tacky but not wet when you pour the SLC — often described as feeling like the back of a sticky label. Check the manufacturer's TDS for the minimum and maximum open times, as these vary by product and ambient conditions.
- In cooler, damper NZ conditions (common in winter in the South Island or Wellington), drying times will be longer.
- In warm, dry conditions, the primer may reach the right tack faster — don't rush it.
- If you pour SLC over wet primer, you risk bubbling and delamination. If you wait too long and the primer fully cures and loses tack, you may need to apply another coat.
Step 6 — Final Check Before Pouring
Before mixing your SLC, do a quick walkover:
- Check the primer is uniformly tacky with no wet or fully dry patches.
- Confirm the room temperature is within the SLC manufacturer's recommended range (typically 10–30°C).
- Ensure doors and windows are closed to avoid draughts, which can cause uneven curing.
- Have all your SLC materials, mixing equipment, and tools ready to go — once you start pouring, you need to work quickly.
When to Call a Professional
While priming and self-levelling can be a DIY project for small, straightforward areas, there are situations where it's worth bringing in a qualified flooring installer:
- Large areas — SLC needs to be poured and spread quickly; managing this solo over a big floor is very difficult.
- High moisture readings — if your slab has elevated moisture, a professional can advise on moisture barriers and appropriate systems to avoid costly failures.
- Underfloor heating — SLC is commonly used over hydronic or electric underfloor heating systems. Incorrect application can damage heating elements. Check MBIE Building Performance guidance and consult a licensed tradesperson.
- Commercial or rental properties — landlords should be aware of Tenancy Services Healthy Homes standards around flooring.
You can find experienced, local flooring installers on tradedirectory.co.nz.
