Kerakoll Primer Uni adhesion-promoting primer for mortar and plaster

Is the render peeling off smooth concrete? On a dusty wall, the fresh layer comes off together with the dust. The adhesion-enhancing primer adheres to both absorbent and non-absorbent substrates. It binds dusty surfaces and regulates water absorption. Its adhesion to concrete is ≥ 3.0 N/mm² after 7 days. This is Kerakoll Primer Uni.

• Smooth, non-absorbent substrate under render: forms a bonding layer — on non-absorbent substrates, pull-off strength ≥ 2.0 N/mm² after 24 hours and ≥ 2.5 N/mm² after 7 days
• Part of the system: for use under mineral mortars, cement and gypsum renders, skim coats and screed compounds (Active Prime Fix for thin self-levelling compounds) — also under Geolite mortar on dry substrates for concrete repair Geolite
• Dusty, soft, weak surface: strengthens and consolidates the surface — prevents surface dusting of render and masonry
• Old concrete, new pour: prepares old concrete — improves adhesion between the old and new layers
• Very porous substrate: reduces and regulates water absorption — diluted with water at a ratio of up to 1:4, it also binds dust
• Dense gypsum and anhydrite substrates: when undiluted, it forms an insulating layer — neutralises gypsum’s volume-expansion reaction when in contact with mineral mortar
• Tight schedules: dries quickly — once the film has hardened (≈ 30–60 minutes at +23 °C), the next layer can be applied, no later than within 24 hours
• Renovation of occupied buildings: EC 1 plus and A+ — water-based, with very low VOC emissions
• Not a finished surface on its own: mortar, render or skim coat must be applied over it; for use under tile adhesive and thin self-levelling compounds, or on tile or stone substrates, use Active Prime Fix

What is it suitable for?

Substrates (absorbent and non-absorbent): Power-troweled concrete floor; precast and monolithic concrete; cement- and gypsum-based plaster; brick-gypsum panel and drywall; wood flooring; rigid metal substrate; varnished surface.

What can be applied on top: Mineral mortars; cement- and gypsum-based plasters; skim coats and floor leveling compounds.

Tasks: Improving adhesion; bonding old and new concrete; regulating absorbency; consolidating powdery surfaces; isolating gypsum and anhydrite substrates.

Areas: Exterior and interior — homes, public buildings, renovations, concrete repair.

Not suitable for:

• As a standalone surface (without mortar, plaster, or skim coat)
• For substrates subject to significant deformation or thermal expansion
• For damp substrates
• For loose, crumbling areas — these must be removed first
• Mixed with sand, binder, or aggregate

The system in which it is used

Which primer is needed? — Deep primers and surface consolidators

Frequently asked questions

How can you prevent plaster from detaching from smooth concrete?
With a bonding primer that forms an adherent layer even on non-absorbent surfaces. On smooth, non-absorbent concrete, use it undiluted; on dense, low-absorption concrete, use it undiluted or diluted 1:2. Apply the plaster after the film has cured (≈ 30–60 minutes at +23 °C), within 24 hours at the latest.

What can be done if the wall is crumbling and shedding sand before plastering?
Remove loose parts and bind the remaining dust with primer. Primer Uni consolidates the soft surface and prevents dusting. To fix dust, it can be diluted with water at a maximum ratio of 1:4.

Is priming required between old concrete and a new pour?
Primer Uni is also suitable for this: it prepares the surface of old concrete so that the new layer adheres better. For a structural construction joint, follow the designer's specification. It can also be used under concrete repair mortar on a dry substrate.

Does the primer need to be diluted?
Not on non-absorbent surfaces or for isolating gypsum, but yes on porous substrates. On dense, low-absorption substrates, use undiluted or diluted 1:2; on very porous substrates, dilute up to 1:4 (primer : water). On dusty gypsum, first dilute up to 1:3, then apply an undiluted coat.

How soon can it be plastered?
≈ 30–60 minutes later, once the film has fully cured (+23 °C, 50% humidity). Apply the plaster between 30–60 minutes and 24 hours later. Drying takes longer in cold conditions.

Primer Uni or Active Prime Fix?
Primer Uni is intended for use under plaster, mortar and smoothing compounds, and between old and new concrete. Under tile adhesive and thin self-levelling compounds, and on tile, stone, PVC or epoxy substrates, use Active Prime Fix. Both adhere to absorbent and non-absorbent substrates.

How much primer is needed per m²?
≈ 0.1–0.2 kg/m² in one coat. One 5 kg bucket is sufficient for ≈ 25–50 m². Two coats are required on dusty gypsum substrates.

When should I not use it?
On substrates affected by moisture, subject to significant deformation or thermal expansion. It should not be used on loose, crumbling surfaces without prior preparation either. Do not mix in sand, binder or aggregate.

Can it be purchased separately?
Yes. Even if the plaster or mortar is already in place, if you are repairing only one section of a wall, or if the quantity proves insufficient.

Can I apply it myself?
The product is intended for professionals, but application is simple: apply evenly with a roller or brush, then crosswise. Dilution depends on the substrate - if in doubt, consult a Masterworks expert. We also undertake plastering and concrete repairs.