Primers for Andura smooth coatings

Classic Smooth and Pro-Flex can be used on sound, dust-free surfaces without a primer. Both are highly resistant to the alkalinity of cement renders.

The problem comes when surfaces are chalky, dusty, or very friable.

A chalky, dusty, or very friable surface.

Classic Smooth is self-priming; it will bind together any slight underlying chalkiness. (The use of the original TMC Primer with Classic Smooth was generally confined to extremely absorbent surfaces.  These surfaces tend to absorb a large amount of binder out of Classic Smooth resulting in variations in surface appearance).

Pro-Flex, in common with water-based coatings in general, may not bind well to very dusty, chalky surfaces. Therefore choosing a primer suitable for Andura's smooth coatings is crucial.

Uk rendered house.

Hydro-seal Primer is designed to penetrate dusty or very friable surfaces resulting in a sound, hydrophobic surface on which to apply coatings.

There is no reason why it couldn't be used with Classic Smooth, but only on very absorbent surfaces where it will penetrate and not leave a surface film.

New Render, there shouldn't be a problem! When it comes to coating new render/cement repair compounds without a primer, problems occur often as a result of poor mix design (not enough cement) or excessively rapid drying (very hot sunny conditions/ very warm breezes) so that the cement doesn't have time to hydrate.  This leaves a sandy, dusty surface.  Too much remaining underlying moisture can also be a problem if not allowed sufficient time to dry out.

Products such as GRC Render are polymer-modified and provide an excellent surface for coating if installed correctly.