In addition to chemical impregnation by pressure-treating, the burning characteristics of wood products can be reduced by applying specially formulated coatings to wood surfaces. These coatings, are generally used for architectural woodwork applications where appearance is important.

Fire-retardant coatings are available in clear and white finishes. Where a solid colour is required, one or two coats of alkyd paint can be applied over the clear or white fire-retardant coating with only a small increase in the flame-spread rating.

The reaction of these coatings to fire and the actual mechanism of protection varies according to the compostion of the coating. Some of the basic mechanisms of protection are as follows:

• Insulation: thick coatings insulate the treated material against high temperatures.
• Crust formation: the coating melts under the action of heat, covering the treated material with an impermeable insulating crust that deprives the wood of oxygen.
• Heat absorption: the coating absorbs the heat and maintains the temperature of the protected surface below its ignition temperature.
• Intumescent insulation: the coating swells when heated to form a thick insulating layer that delays the spread of flame and the transmission of heat to the protected surface.

Like FRTW products, wood products protected by fire-retardant coatings, because of their reduced flame-spread rating, can be used in areas where untreated wood products cannot be used. However, most fire-retardant coatings are not suitable for use in high humidity or exterior applications.

Fire-retardant coatings are manufactured as proprietary products subjected to extensive fire testng. For specifications on rate of coverage and properties, a manufacturer should be consulted.

These products can be applied by brush, roller, or sprayer. Because fire-retardant coatings are high viscosity (thick) liquids, they should be maintained at room temperature, especially when spray applied, to ease application. Where appearance is important, two light coats , by reducing sagging, are superior to one heavy coat and provide the required flame-spread characteristics.

Fire-retardant coatings can be used for new construction, and for rehabilitation projects, to give surfaces burning characteristics which meet modern requirements. For previously uncoated surfaces, the normal surface preparation required for paint, stain, varnigh, and lacquer will ensure good adhesion and performance. For prefinished panels and for previously painted, waxed or otherwise coated surfaces, light sanding is required followed by application of an undercoat specially formulated to provide the adhesion between the surface and subsequent fire-retardant coating.