Additives affecting chemical reaction can be described as ’activators’ (or catalysts). 'Accelerators', ‘driers', 'inhibitors' or ’retarders'. These may be categorised:

1) Those that initiate the drying reaction.

2) Those that affect the rate of drying.

An activator may be one portion of two pack paint, one reactant of the resin-forming ingredients. In this instance, the activator can hardly be called an additive. The true additive activator is a chemical which when added as a minor ingredient, sparks off the chemical reaction in the paint.

Such an additive is usually a chemical which decomposes to give free radicals which, in turn, initiate an addition polymerisation e.g. peroxides.

Driers and accelerators are true catalysts in the chemical sense, since they can speed up the chemical reaction responsible for drying without being consumed in the process.

Driers are typically present at less than 1%. Primary driers are true catalysts and contain metals of variable valency. The lower valency being the more stable one, yet capable of oxidation to the higher valency by the products of the drying process. Cobalt and Manganese are typical driers of this type. The metals act in two ways:

1) They catalyse the uptake of oxygen.

2) They catalyse the decomposition of peroxide to free radicals.

While Cobalt and Manganese carry out both functions, Lead only catalyses oxygen uptake. Lead is a secondary drier, one of a group of metals, including Zirconium, Calcium and Cerium, which assist the drying of the lower layers of paint by mechanisms not fully understood.