A ‘Drier’ is a substance which acts as a catalyst and accelerates the process of drying of a paint film. A drier absorbs the oxygen from the atmosphere and becomes a higher oxide. This drier then in turn oxidizes the vehicle, i.e., linseed oil, which gets hardened. Thus, the only function of driers is catalytic. However, the driers adversely affect the colour and elasticity of the paints. They are, therefore, not used in preparing the paint for the final or finishing coat otherwise they will lead to flaking of the paint.

Driers are classified into two types:

(i) Oil driers (or soluble driers): These are the driers which are dissolved in linseed oil or some other good oil. These are available in powdered form, e.g., litharge, magnesium dioxide, magnesium borate, etc.

(ii) Paste driers: These driers are first mixed with inert fillers, such as barites, whiting, silica, etc. and then ground in linseed oil or some other good oil, e.g., cobalt or vanadium compounds.
Out of above driers, Litharge is the most commonly used drier. Red lead and sulphate of manganese can also be used as driers.

It should be noted that a drier more than 10% by volume of the paint should not be added and moreover, only one kind of drier and that too just before using the paint, should be added, if necessary.