Preparation of a primed soil/aggregate base surface or an existing asphalt-surfaced pavement for an overlay typically includes an underseal which is a sprayed application of asphalt binder (asphalt emulsion or hot applied asphalt binder) immediately covered by a layer of single-sized aggregate. The underseal provides several benefits, such as waterproofing the surface, sealing small cracks, and protecting the underlying surface from solar radiation. The application of an underseal should be strongly considered when placing a thin ACP layer on top of a flexible base. The covers in detail the design and construction of seal coats.