Paint is the most commonly used material to protect steel. Paint systems for steel structures have developed over the years to comply with industrial environmental legislation and in response to demands for improved durability performance. Previous five and six coat systems have been replaced by typically three coat alternatives.

Modern specifications usually comprise of a sequential coating application of paints or alternatively paints applied over metal coatings to form a ‘duplex’ coating system.

Composition of Paints and film formation

Paints are made by mixing and blending three main components:

  1. The pigments
    Pigments are finely ground inorganic or organic powders which provide colour, opacity, film cohesion and sometimes corrosion inhibition.

  2. The binder
    Binders are usually resins or oils but can be inorganic compounds such as soluble silicates. The binder is the film forming component in the paint.

  3. The solvent
    Solvents are used to dissolve the binder and to facilitate application at the paint. Solvents are usually organic liquids or water.

Paints are applied to steel surfaces by many methods but in all cases this produces a 'wet film'. The thickness of the 'wet film' can be measured, before the solvent evaporates, using a comb-gauge. As the solvent evaporates, film formation occurs, leaving the binder and pigments on the surface as a 'dry film'. The thickness of the 'dry film' can be measured, usually with an electro-magnetic induction gauge. The relationship between the applied 'wet film' thickness and the final 'dry film' thicknesses is determined by the percentage volume solids of the paint, i.e. dft = 'wet film' thickness multiplied by the % vol. solids.

In general the corrosion protection afforded by a paint film is directly proportional to its dry film thickness.

Paint constituents

Schematic diagram of a Paint film

The method of application and the conditions under which paints are applied have a significant effect on the quality and durability of the coating. Standard methods used to apply paints to structural steelwork include application by brush, roller, conventional air spray and airless spray.