INTERIOR SURFACE PREPARATION

CONCRETE AND MASONRY

New concrete must be allowed to age a minimum of 28 days prior to painting. All dirt, grease, release agents, rust and foreign matter must be removed from the surface. Stains that have penetrated the surfaces may require sandblasting for removal. The pH of the concrete surface should be within the 7 to 8.5 range for safe coating application (This is a very simple test and instructions for use and a pH number colour code chart are normally included with a indicator paper kit). If the surface is not within this range or large amounts of efflorescence are present then the concrete should be acid etched prior to painting. Any large holes or cracks should be filled with a concrete patching compound and allowed to cure completely. The concrete must be dry prior to coating (less than 12% moisture content).

Efflorescence is seen as a white salty deposit on the concrete surface. This condition is caused by moisture in or entering the surface, which causes water-soluble salts in the masonry to be brought to the surface and as the water evaporates the residual salts are left as a surface deposit.

Latex coatings are typically preferred because they have superior breathability and alkali resistance than solvent-based coatings. When painting concrete with a latex coating the concrete can be damp before painting but not wet. In fact, a light misting of the concrete with water prior to painting will help the paint to penetrate into the concrete.

If a solvent based coating is required it is recommended that either the first coat be thinned and applied by brush to promote penetration into the concrete, or if possible a latex sealer be used first to reduce the exposure to alkali conditions.

Steps to etch a concrete surface.

1. Clean the concrete of all soil including oil and grease (a T.S.P. solution of 50 grams per litre will remove most oil and grease) and thoroughly rinse with clean water.

2. While the concrete is still wet apply the etching solution (one part Commercial Muriatic Acid and three parts water) and distributing this uniformly by plastic watering pail.

3. Sweep surface with a synthetic bristle broom to ensure uniform treatment.

4. Thoroughly rinse the concrete with clean water until the pH is in the range 7.0 to 8.5 and wait until the concrete is completely dry before painting.

(Caution Note: When using the Muriatic acid follow all safety precautions and refer to the Material Safety Data Sheet for complete Health and Safety information.)

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