The spray-on method is used to apply a stencilled finish to existing suitable substrates, such as concrete or tarmac. As there is no excavation and casting of a new slab, this method is considerably cheaper than the wet cast method and can be used to completely rejuvenate old, tired surfaces, provided they are structurally sound. (Using the spray on system over cracked or crumbling concrete is a complete waste of money; it is a surface dressing, not a repair system.)
The existing surface must be carefully prepared prior to applying the base coat and spray-on topping. Concrete surfaces should be power washed and/or acid-etched to ensure a good bond between the base coat and the existing concrete. Tarmac surfaces also should be power-washed to remove all detritus and then sealed with a special primer coat. Access and drainage fittings in the surface should be suitably masked off before applying the coatings.
Once primed, the colored base coat can be applied. This usually is a high-strength cement-based coating that is spread using a float or a squeegee to cover the surface evenly. It dries rapidly to ensure the work can progress with minimal interruption and wasted time for the contractor.
Next, the chosen stencil is laid out over the base-coated surface. It is laid flat so that it adheres to the base coat, and checked for alignment. The stencil mask used for spray-on applications is exactly the same as that used for the wet-cast system. A contrasting pattern usually is chosen for the edges, often a 'soldier course' pattern. The edging stencils are laid out first, and then the main pattern stencils, which are neatly trimmed to fit in with the edge stencils.
The top coating is a high-strength cement based powder that is mixed with the chosen color dye from a range of around thirty different shades. The mixed color topping is fed to a hand-held hopper with the aid of a small air compressor and then carefully sprayed over the surface in a number of sweeps to achieve an even coverage.
Once the top coat has dried (usually a couple of hours), the stencil is simply peeled away to reveal the pattern. Finally, after 2-4 days, a quality two-coat resin sealant is applied to enhance the color and protect the surface from accidental staining. This sealant, like most others used for sealing decorative concrete, must be re-applied every 2-4 years to keep the surface in best condition.