FINISHES APPLIED AFTER MANUFACTURE ? PAINT FINISHES

A typical texture system for precast concrete wall panels will involve three steps:

  1. Primer
  2. Texture
  3. Finish Coat

1 - Primer

Primer is the most important part of any system. It provides a sound base for adhesion of top coats. Water and solvent based primers are used. Water-based primers are most widely used for their convenience - low odour, water thinning and wash up.

Solvent-based primers are used on "difficult" surfaces eg impervious, smooth surfaces or those with residual release agents.

Water-based primers (and other water-based paints) contain emulsion as binder. Emulsions are discrete particles of resin dispersed in water. For this reason they do not penetrate smooth, impervious surfaces. In addition release agents can act as water repellents making water-based primers unsuitable for such surfaces.

Solvent-based primers have the resin in solution. Solvent can dissolve release agents allowing the primer to penetrate and gain adhesion.

In summary, solvent-based primers are preferred for precast as they are able to easily penetrate smooth dense surfaces to provide a sound base for adhesion of topcoats. In addition, they are fast drying with a two hour recoat and have excellent alkali resistance.

2 - Texture

There is a wide range of products with a variety of builds and profiles.

Roller Applied. - Roller application is quick and easy and does not require highly skilled applicators. "Stipple" finishes are the simplest systems requiring only primer and one or two coats of texture.

Trowel Applied. - Application is slower and requires skilled applicators. After trow elling, the coating surface is worked with a polystyrene float to achieve the traditional scratch effect. Trowel coatings are available in 1, 1.5, 2 and 3 mm grades. Other trowel-applied products do not have coarse aggregate and leave a smooth render look.

Spray Applied. - Textures can be applied with a hopper gun or spatter gun to achieve a "stucco" effect. Spray application is fast so application costs are reduced but some skill is required.

3 - Finish Coat

Finish coats provide colour, dirt shedding and long-term decorative appearance. They are applied with a conventional nap roller and are often referred to as "impact" coatings. Applied with a normal roller they provide a long life decorative topcoat over trowel or other textures.

For further guidance on precast concrete surface preparation prior to paint application, reference should be made to AS 3610-1995 Formwork for Concrete and AS 2311-1998 Guide to the Painting of Buildings.

Acknowledgement is made to National Precast Concrete Association Australia for use of material on paint finishes.

FINISHES APPLIED DURING MANUFACTURE

Bricks and tiles can be cast onto precast concrete although this has not often been done in Australia. Techniques are developed to suit each project but usually sufficient bond can be achieved between the bricks and the tiles to give a permanent finish (see Product Application).

More commonly used are natural stone facings. Thin slabs of natural stone are placed in the mould to a pattern and structural grade concrete poured behind. The stone veneer is connected to the concrete backing with a series of stainless steel dowels. These are bonded into the stone by drilling into the back face of the stone veneer and securing with an epoxy adhesive. The dowels are inclined to provide a dovetail effect. The interface between stone and concrete is debonded to allow for differential thermal and moisture movement of the two materials.