A prime coat is the application of a low viscosity binder, usually a bituminous emulsion, to a gravel, WBM or WMM surface, to enable the binder to fill up the voids. The rate of application is 0.6-1.5 Kg per sq m of the surface, the quantity depending upon the porosity of the surface. The application is done by a mechanical sprayer or by hand operated spraying lance.
The tack coat is a bituminous application over a primed granular surface or an existing bituminous surface to promote a bond between it and the bituminous layers coming above. The tack coat is normally done with a bituminous emulsion at a rate of 0.2 to 0.45 Kg per sq m, depending upon the type of surface. Like a prime coat, the tack coat is applied by spraying with a mechanical sprayer or a hand held lance.