When render peels or tiles let go, the material usually gets the blame. In most cases the material was right and the surface was never prepared. Preparation comes down to a single question.

The only question: absorbent, or smooth?

These are opposite problems with opposite cures, and applying one to the other makes things worse.

1) Absorbent — the problem: it steals the mix water

Block, brick, porous concrete and gypsum pull water out fast. Cement needs that water to hydrate and gain strength; drawn away too early, the mortar dries before it cures — friable, prone to peeling and hairline cracking.

Cure: a penetrating acrylic primer — it soaks into the pores, consolidates them and regulates absorption.

2) Smooth — the problem: nothing to grip

Formwork concrete is closed and slick, sometimes with release-oil residue. Mortar finds no pores and sags off.

Cure: Betokontakt — a quartz-filled contact primer that creates an artificial key for mechanical grip. Usually pink so you can see it was applied.

The water test — thirty seconds

  • Vanishes instantly, leaves a pale patch → absorbent → penetrating primer.
  • Beads and sits on top → smooth → Betokontakt.
  • Slow, moderate absorption → a thinned penetrating primer is usually enough.

More reliable than judging by eye: plenty of surfaces look rough but are closed, and vice versa.

The common error: swapping them

  • Betokontakt on an absorbent surface is drunk into the pores and disappears — money spent, absorption unchanged.
  • Penetrating primer on smooth concrete has no pores to enter and can leave a slick film that makes adhesion worse than bare.

The right sequence

  1. Clean — dust, oil and loose material. A loose old layer takes your work down with it.
  2. Repair — fill large defects first; primer does not fix a failing substrate.
  3. The right primer, per the water test.
  4. Wait. The step everyone skips. Rendering over damp primer traps moisture and weakens the bond. Read the drying time on the pack and respect it.
  5. Render, then level — see mortars & levelling.

Special cases

  • Gypsum: very absorbent and moisture-sensitive. Penetrating primer is mandatory; no heavy cementitious layers over it in wet areas.
  • Bathrooms: a primer is not waterproofing. One prepares the surface, the other stops water — two different jobs.
  • Façades: daily thermal movement means a rigid material will crack however well prepared.
  • Old painted surface: if the paint is sound, abrade it then Betokontakt; if it flakes, remove it entirely — no primer rescues a loose layer.

In short

Primer is not the line item to cut. A bag of adhesive on an unprepared surface is money thrown away, because the failure does not show on the day — it shows months later, when the repair costs many times the primer.

Compare primers in construction chemicals, or start from the full catalogue.