Levelling and finishing compounds for concrete: pore fillers and putties

Why a levelling coat is needed

After repair with repair mortars, and on new cast-in-place concrete too, the surface is rarely ready for painting: there are blowholes, steps between the patches and the old concrete, spalled arrises and formwork marks. Levelling and finishing compounds cover these defects with a thin layer — from fractions of a millimetre to several millimetres — and create a smooth, dense substrate for protective coatings.

Pore filling is not cosmetic. Blowholes under a coating are places where the film ends up thin or discontinuous, and when the surface heats up in the sun, air escapes from the pores and forms blisters. Manufacturers of protective coatings explicitly require blowholes and pores to be filled beforehand: without this, the declared protection is not achieved.

Types

Polymer-modified cementitious finishing mortars

Fine-grained one-component cement-based mixes with polymer and silica fume. Under EN 1504-3, they fall into the non-structural class R2 or, if their properties allow, into class R3. They are used to level repaired areas and cast-in-place concrete, fill blowholes and restore minor spalls at corners and joint arrises. They are applied with a spatula, a trowel or by spraying, in a thin layer within the limits stated in the data sheet. Examples: Sika MonoTop-620, Fosroc Renderoc FC, Mapefinish.

Epoxy-cement compounds

Three-component mortars in which the cement is modified with epoxy resin. They are denser and more chemically resistant than polymer-modified cementitious mortars, bond well to damp and even young concrete, and are impermeable to liquids but permeable to vapour. Applied in a layer of sufficient thickness, such a compound acts as a temporary moisture barrier and allows epoxy and polyurethane coatings that need a dry substrate to be applied sooner. Example: Sikagard-720 EpoCem.

Epoxy putties

Thin epoxy compounds for filling pores and irregularities and for finishing surfaces where resistance to abrasion or chemicals is needed, as well as for bonding packers and sealing cracks before injection. Example: Fosroc Nitomortar FC.

How to choose a compound

  • Thickness. Each product has a range of thickness per pass — from fractions of a millimetre for putties to several millimetres for finishing mortars. Deep defects are first filled with a repair mortar, and the surface is then brought to a finish with a levelling coat.
  • Substrate moisture. Polymer-modified cementitious mortars are applied to a saturated substrate without puddles; epoxy-cement compounds tolerate damp and young concrete; epoxy putties need dry or matt-damp concrete — as stated in the product data sheet.
  • What goes on top. A cementitious finishing coat is suitable under vapour-permeable acrylic coatings; under vapour-tight epoxy and polyurethane systems on damp concrete, an epoxy-cement barrier coat is used, and the coating is applied over it once the surface moisture has fallen to the value given in the data sheet.
  • Environment. In a chemically aggressive environment, epoxy-cement or epoxy compounds with confirmed resistance are chosen; we confirm resistance to a specific substance by our questionnaire.
  • System. The finishing coat must be compatible both with the repair mortar beneath it and with the coating above it. The simplest approach is to take all the layers from one manufacturer: the data sheets state explicitly which levelling compound goes with which repair mortar.

Levelling under polymer coatings on damp concrete

A common task is to apply an epoxy or polyurethane coating to concrete that has not yet dried out: young cast-in-place concrete, structures with capillary moisture, basements and tanks after testing. Ordinary polymer-modified cementitious mortars do not help here — the coating still needs a dry surface. Epoxy-cement compounds solve the problem differently: they are applied to a matt-damp substrate without standing water, and a layer of sufficient thickness — for Sikagard-720 EpoCem, at least 2 mm — acts as a temporary moisture barrier. The vapour-tight coating is applied once the surface moisture of the epoxy-cement layer has dropped to the value stated in the data sheet. The barrier effect is limited in time: if more than a few days have passed since application, the surface moisture is checked again. Under PMMA-based coatings, the manufacturer requires the layer to be fully broadcast with sand of 0.4–0.7 mm grading.

Procedure

  1. Preparation. Laitance, traces of release agent, old coatings and any weak layer are removed mechanically — by abrasive blasting or scarifying; blowholes are opened up so that the compound fills them completely. Dust is removed with an industrial vacuum cleaner or a brush.
  2. Pre-wetting. Under cementitious compounds, the substrate is saturated with water, and before application the excess water is removed until the surface looks matt-damp.
  3. Mixing. Strictly in accordance with the data sheet: a measured amount of water for one-component mixes, complete kits for multi-component products; a low-speed mixer, so as not to entrain air.
  4. Application. With a spatula or trowel, pressing the material into the pores; on large areas — by spraying or with special equipment, followed by hand finishing. For a seamless surface, work “wet edge to wet edge”.
  5. Finishing. With a sponge or a plastic float when the layer begins to stiffen — without adding water.
  6. Protection of the fresh layer. From sun, wind, rain and condensation — for the periods given in the data sheet.

Common mistakes

  • A levelling compound used instead of a repair mortar: a layer that is too thick for one pass cracks and debonds.
  • Application to a dusty, uncleaned or dry substrate.
  • Water used during finishing — stains, fine cracks, a weak surface layer.
  • Outdoors — application while the temperature is rising during the day: air escapes from the pores and leaves “pinholes” in the surface; manufacturers recommend working while the temperature is falling.
  • A vapour-tight coating over a cementitious layer that is still damp — blistering and delamination.
  • Moving cracks covered with a rigid thin layer: the crack reappears at the surface. Such cracks are assessed on site and, where necessary, covered with an elastic material, or a movement joint is formed.

What to check with the supplier

  • the class to EN 1504-3 and, for epoxy-cement systems, compliance with EN 1504-2 as well;
  • the minimum and maximum layer thickness, the permissible moisture content and age of the substrate;
  • when and with which coatings the layer can be overcoated;
  • the application temperature range and the protection periods for the fresh layer;
  • compatibility with the repair mortar and the protective coating of the chosen system.

We select the make-up of the system — repair mortar, levelling coat, coating — for your project as part of our engineering services; protective coatings are described in the “Protective coatings and impregnations” section.

Materials in the catalogue

  • Sika MonoTop-620 — Sika — class R3 levelling and finishing mortar for blowholes and surface reprofiling.
  • Sikagard-720 EpoCem — Sika — epoxy-cement micro mortar for levelling and pore sealing, even on damp concrete.
  • Fosroc Renderoc FC — Fosroc — R2 cementitious fairing coat for filling blowholes up to 3 mm.
  • Fosroc Nitomortar FC — Fosroc — epoxy fairing coat for filling concrete blowholes before epoxy coatings.
  • Mapefinish — Mapei — two-component R2 mortar for thin levelling and protection of concrete.
  • Skrepa Finishnaya — Penetron — fine B25 W18 finishing mortar for defects up to 10 mm and levelling.
  • Monofinish — Mapei — polymer-modified R2 levelling and protective mortar for concrete, 1–3 mm layers.
  • Triblock Finish — Mapei — epoxy-cement mortar for levelling damp concrete before epoxy and PU coatings.

This list is not exhaustive. The final choice is made on the basis of the manufacturer’s documents and the project requirements.

Frequently asked questions

Can concrete be levelled with a plastering mix?

For interior finishing, yes, but protective coatings for concrete need compounds that have been tested for bond to concrete and compatibility with the coating. Ordinary plaster may come away together with the paint and does not protect against carbonation.

Do all blowholes have to be filled before painting?

Yes, if the coating is to perform a protective function. Open pores lead to thin spots in the film and to blisters; coating manufacturers require a continuous, defect-free substrate.

How soon can a levelled surface be painted?

As stated in the data sheets of the levelling compound and the coating. On an epoxy-cement layer, vapour-permeable coatings are applied once it is tack-free, and vapour-tight ones once the surface moisture has dropped to the value specified by the manufacturer; cementitious compounds need more time.

We will select a levelling compound and a compatible coating — call us or message us on WhatsApp/Telegram: +998 94 094 52 27.