Floor Priming Before Levelling

Every subfloor absorbs water differently. Old concrete soaks it up like a sponge, while smooth anhydrite screed takes in almost nothing. If you pour levelling compound onto an unprimed surface, the absorbent patches draw water out of it quickly. In those spots the compound can't cure properly. It ends up weaker and may crack or come loose later.

Primer does three things:

• It evens out absorbency, so the compound dries at the same rate across the whole floor
• It binds leftover dust and strengthens the top layer of the subfloor
• It creates a bond between the subfloor and the new layer

Manufacturers of levelling compounds usually list priming as a requirement in their technical data sheet. If you skip it, a warranty claim becomes much harder.

This job usually comes in two versions. They differ in how much gets done before the compound goes down.

Priming only means cleaning the floor and applying primer. It suits rooms where the walls and floor meet tightly, or where a perimeter edge strip is already fixed around the room and there are no open gaps anywhere.

Priming with sealed corners adds a step: corners, gaps along the walls, openings around pipes and gaps at door thresholds are sealed with acrylic sealant. Levelling compound is very runny and will find every opening. In practice it often seeps under a wall into the next room or into the void beneath a threshold. In a block of flats it can run through a gap around a pipe to the floor below. Sealing these spots keeps the compound where it belongs, and the layer ends up as thick as planned.

This is a different job from running sealant along skirting boards before painting. That happens at the very end, once the floor covering and skirting are fitted, and it's mainly about appearance.

The right primer depends on what you're pouring onto:

• Universal primer for firm subfloors with normal absorbency, such as newer concrete or sand and cement screed, often sold ready to use
• Deep-penetrating primer for old, dusty or slightly crumbly subfloors, as it soaks in further and consolidates the surface
• Primer for anhydrite and gypsum screeds, which are sensitive to moisture and need a product the manufacturer specifically approves for them
• Bonding primer for non-absorbent surfaces such as old ceramic tiles or terrazzo, where an ordinary primer would have nothing to soak into

The safe choice is a primer and levelling compound from the same manufacturer. The two products are tested together, and the data sheet tells you exactly how to dilute the primer, how much you need and how long it takes to dry.

Most of the time goes on preparation, not on the primer itself. These things make the job take longer:

• Old adhesive residue that has to be ground off or scraped away
• Greasy patches, typically in former kitchens and workshops
• Crumbly areas that need cutting out and repairing
• A very absorbent subfloor that needs two coats
• An awkward room with many corners, pipe openings and thresholds, where a lot of the work is done by brush and, in the sealed version, with sealant

A large, regular area of clean screed is quick to prime, usually with a roller alone.

You can prime a floor yourself. The job isn't dangerous, but it needs care and patience while the primer dries. If the subfloor seems doubtful, for example crumbly or damp, ask a professional to look at it first.

1. Remove adhesive residue, old coatings and loose material with a scraper or grinder.
2. Degrease oily patches with a suitable cleaner and let them dry.
3. Vacuum the whole floor thoroughly, ideally with an industrial vacuum cleaner.
4. Check that the subfloor is dry and firm. If in doubt, have the moisture content measured.
5. Fix a perimeter edge strip around the room if there isn't one already.
6. For the sealed version, seal corners, wall gaps, pipe openings and thresholds with acrylic sealant and let it set as directed.
7. Mix the primer according to the data sheet. Don't guess the dilution.
8. Brush the primer into corners, along edges and around pipes.
9. Roll the main area in a criss-cross pattern, starting from the corner furthest from the door and working towards it. Keep the coat thin and leave no puddles.
10. On a very absorbent subfloor, apply a second coat once the first has dried.
11. Let the primer dry as the manufacturer specifies, usually several hours, and keep off the floor until then.

Work at roughly 10 to 25 °C, out of draughts and direct sunlight. In the cold the primer takes much longer to dry. In the heat it can dry before it has soaked into the subfloor.

• Priming over dust or grease: the primer bonds to the dirt instead of the subfloor, and the levelling layer later peels away
• Missed spots, especially along walls and behind radiators, where bubbles or hairline cracks later appear
• Too thick a coat or puddles: these leave a glossy film that the compound grips less well
• Diluting by eye: primer that is too thin doesn't work, and primer that is too thick forms a sealed skin
• Pouring onto wet primer: the layers don't bond, and the floor may crack later
• Unsealed gaps: the compound runs into voids or down to the neighbours, and the room ends up short of material

Before anyone starts mixing the compound, go over the floor:

• The surface is dry and no longer tacky to the touch
• The coat is even across the whole floor, with no missed or glossy patches
• Corners, edges and the areas around pipes are primed
• A perimeter edge strip runs around the whole room
• In the sealed version, corners and gaps are sealed all the way along and the sealant has set
• There is no fresh dust and there are no footprints on the floor
• The primer used matches the one the manufacturer recommends for the chosen levelling compound