A liquid reservoir of a hand-held, air-assisted spray apparatus is characterized by a plurality (i.e., two or more) of liquid-holding compartments. Each liquid-holding compartment is connected through an outlet port into fluid communication with the flow channel of the hand-held spraying apparatus.The liquid-holding compartments are isolated from fluid communication with each other such that a liquid introduced into one of the compartments is held apart from a liquid introduced into the other compartment.A rotatable element is associated with each liquid-holding compartment. Each rotatable element has a predetermined displacement. The rotatable elements are coupled together for rotation by a common shaft, so that, in use, the relative volumes of liquids dispensed from the respective liquid-holding compartments into the flow channel remain in a predetermined ratio regardless of the rate at which the liquids flow from their respective compartments. The component formulations of a multi-component coating such as that based on hydroxyl/isocyanate chemistry for automotive clearcoats are kept isolated until the actual time of application.