Collection: Special Primer

There are substrates for which neither deep primer nor bonding bridge provides an adequate solution — because the substrate material has special chemical properties that require different treatment. The most common cases are calcium sulfate (anhydrite) screed, drywall, and previously painted surfaces treated with chemical cleaners.

Preparation of Calcium Sulfate and Anhydrite Screed

Calcium sulfate (anhydrite) screed — a substrate that looks similar to traditional cement screed but has completely different chemical behavior — is one of the most common sources of incorrect substrate preparation decisions. It is not directly compatible with cement-based tile adhesives and standard primers: chemical incompatibility disrupts the bonding process, leading to early failure of the adhesive. Calcium sulfate screed should only be prepared with a specially certified primer and covered with an adhesive system compatible with it.

To identify calcium sulfate screed on site: a water test (the surface darkens immediately and hardly absorbs water) and a scratch test (it is not scratched as easily as gypsum) help. In uncertain cases, the contractor’s documentation should be obtained.

Drywall Preparation

Ceramic tiles can only be installed on drywall with proper preparation. The drywall surface is absorbent, but the paper surface and gypsum core behave differently — especially when wet — which can cause problems in the adhesive curing process. A special drywall primer balances the absorbency and strengthens the paper-adhesive bond. In wet rooms (showers, walls next to bathtubs), drywall must be water-resistant and used only together with the waterproofing system prescribed by the manufacturer.

Preparation of Painted and Chemically Treated Surfaces

Previously painted walls, surfaces treated with industrial cleaners, and impregnated stone surfaces also require special priming. Standard primers do not penetrate the paint layer and do not bond properly to chemical residues. The special adhesion-enhancing primer physically etches and embeds into the old coating, creating a stable surface for the next layer.

When Is a Special Primer Absolutely Necessary?

Use of a special primer is mandatory:

• on calcium sulfate and anhydrite screed before tile adhesive, leveling compound, and screed;
• on drywall in wet rooms before waterproofing;
• on painted, impregnated, or chemically treated substrates before tiling and plastering;
• where the substrate type cannot be reliably identified and cement compatibility is uncertain.

Incorrect substrate preparation decisions on calcium sulfate screed not only lead to tile failure — the entire system must be removed. This results in significantly higher repair costs than proper preparation done on time.

Browse our special primers or request advice to assemble the right preparation for your substrate type and planned system.