Paint can be applied to a surface by means of a spray gun. The spray gun system consists essentially of a small container into which the paint is fed under gravity or by pumping, and from which the paint is forced through a nozzle controlled by a needle valve to give a fine mist or atomizing spray.

This method in various forms and adaptations is widely used, but it is not economical in terms of the consumption of paint, particularly with guns operated by air pressure since the spray does not deposit only on the object being coated, but spreads itself beyond and around, necessitating the provision of special plant to trap the overspray and prevent it contaminating the work atmosphere. The trapped materials can in some cases be used again. Two component spray guns are made for compositions which depend for their hardening process on a chemical reaction activated by catalyst or hardener which is fed from a separate container at a point almost immediately before the spray emerges from the jet. Means are provided for adjusting the mixing ratio and controlling the rate of reaction. These type of guns are used for particularly fast curing materials.