TWO reasons-1. To make it possible to spread the paint evenly and uniformly on the surface in the form of a thin layer2. To provide a binder for the ingredients of a paint so that they may stick or adhere to the surface.NamedescriptionLinseed oilThis is the most common material used as vehicle of a paint. It is extracted from flax seeds. The linseeds oil prepared from fine full grown ripe seeds is clear, TRANSPARENT, pale sweet to the taste and practically odourless. It is used in various grades.Raw linseed oilThe raw linseed oil is thin and pale. It requires more time for drying and is used for interior work of DELICATE nature.Boiled linseed oilThis oil is thicker and darkly coloured than raw oil. It dries quickly. It is prepared by adding some quantity of drier such as litharge or red lead to the raw oil. It cannot be used for interior delicate work. It is used for exterior surfaces.Pale boiled linseed oilThis is similar to boiled linseed oil expect that it does not possess a dark colour. It is more suitable for painting plastered surfaces.Double boiled linseed oilThis oil dries very quickly and is suitable for external work. It HOWEVER requires a thinning agent like turpentineBase-It is a solid substance in a fine state of division and it forms the bulk of a paint. It determines the character of the paint and imparts durability to the surface which is painted.Examples-White lead, Red lead, oxide of zinc or zinc white, Oxides of Iron, Titanium white, Antimony white, Aluminium POWDER, LithophonePigments-The addition of dyes and pigments helps in two ways, namely, they act as fillers and they impart desired colour to the plastics. They should be durable and adequately fast to light.The commonly used pigments are zinc oxides, barytes, etc. the selection of pigments should be done in such a way that their addition does not alter or affect the other properties of plastics.