There are certain structures, such as steel bridges, oil refineries and tank farms etc. where protection is of greater importance than decoration and these surfaces require special types of paint.

Sealers are used when the surface to be painted is highly porous or can b exclude material which will damage subsequent coats of paint. Examples of porous surfaces are asbestos and other building boards, certain types of new plaster and hardboards.

The sealers are chosen to satisfy the suction of the surface and seal off any alkali such as lime. Sealers based on vinyl emulsions are frequently used.

Timber often contains knots which can exude a resinous substance, which will soften and discolour paint films. To avoid this, the knots are treated with a sealer known as “knotting”, which is a solution of Shellac in methylated spirit.

A primer is the first coat of paint to be applied to a substrate and is often designed to perform as both primer and sealer. The function of a primer can be regarded as two fold; a) to give adequate protection and good adhesion to the substrate and b) to provide a good surface for the undercoat. Primers if are formulated according to the type of substrate to be coated.