The unsaturated oils react with oxygen from the air which cause the oils to polymerize or crosslink with each other. The drying speed of the coatings depends on the amount and type of drying oil employed (more unsaturated oil means faster reaction with oxygen from the air) and use of organic metal salts or "driers" which catalyze crosslinking between the unsaturated oils used to modify the base polyester resin.
Traditionally organic lead salts were employed but have been replaced with cobalt and other alternative driers such as zirconium, zinc, calcium, and iron. Most driers are colorless but cobalt is a deep blue purple color (iron driers are reddish orange) thus the colored driers are typically employed only in paint colors where the blue color of cobalt or reddish orange color of iron can be easily masked.
Alkyd coatings are produced in two processes; fatty acid process and the alcoholysis or glyceride process. Higher quality higher performance alkyds are produced in the fatty acid process where the composition of the resulting resin can be more precisely controlled. In this process an acid anhydride, a polyol and an unsaturated fatty acid are combined and cooked together until the final product has achieved a predetermined level of viscosity as suitable for its intended use. More economical alkyd resins are produced from the alcoholysis or glyceride process where end product quality control is not as paramount. In this process raw vegetable oil, high in unsaturated component, is combined with additional polyol and heated to cause transesterification of the triglycerides into a mixture of mono- and diglyceride oils. To this resulting mixture acid anhydride is added to build molecular weight of the resin into roughly the same product as in the fatty acid process. However the alcoholysis or glyceride process produces a more randomly oriented structure. In both cases the resulting product is a polyester resin to which pendant drying oil groups are attached. At the conclusion of both processes the resin is purified, diluted in solvent and sold to paint and varnish makers.
Because the major components of an alkyd coating, i.e. fatty acids and triglyceride oils, are derived from low cost renewable resources this has kept the cost of alkyd coatings very low despite ever increasing cost of petroleum which is the predominant raw material source of most other coatings such as vinyls, acrylics, epoxies, and polyurethanes. Typical sources of drying oils for alkyd coatings are sunflower oil, safflower oil, soybean oil, fish oil, corn oil, and tall oil (resinous oil by-product from pulp and paper manufacturing).