This material is much used in the automotive industry because of relatively cheap tooling, lightness and strength. The process normally involves the application of a gel coat to the surface of a mould, followed by glass fibre matting and (activated) polyester resin. The polyester cures and a rigid structure is formed.

From a paint point of view several problems can occur, the gel coat fig; particularly if hand laid up will contain pin holes associated with air bubbles. The polyester on curing shrinks and because increased thickness of mouldings get hotter due to the chemical reaction, shrinkage is not uniform and may continue for several months or years. Stress due to the shrinkage can cause cracks, particularly from stressed areas, or voids in the structure.

The reaction of polyester may be inhibited by air. This accounts for the sticky surface on the back of the moulding and can result in uncured material in voids which may later break down and blister.

To prevent mouldings from sticking, release agents are used in the mould. This could be P.V.A., bees wax, or flouro carbon release agents. These must be removed from the gel coat before painting.

Steel is a term used to describe a mixture of chemicals mainly dominated by iron and because the painting industry is dependent upon the steel industry, particularly in car manufacture, it is interesting to trace its history.

Typical Victorian iron or pig iron obtained from blast furnace was brittle, mainly because of its high carbon content. In 1855, Henry Bessemer, experimented in his St. Pancras workshop by blowing air through molten pig iron at 1200°C. This process oxidised manganese, silicon and carbon out of the mix as burnt gases or slag.

Leaving the carbon content too low, oxygen left in the melt tended to froth and sulphur in the mix, caused the grains to fall apart when forged. These problems were solved in 1857 when Robert Muschet added an alloy of manganese iron and carbon to the melt after the air blast, thus increasing its carbon content and removing the sulphur as manganese and sulphur globules in the steel and not iron and sulphur.

Within 10 years British Steel was less than one quarter of its 1856 price. In 1872 Andrew Carnegie further refined the process.

Victorian Steel:

a) From blast furnace.

b) After aeration of Bessemer converter.

c) After addition of ferro manganese.

Stage   Carbon            Silicon             Phosphours      Sulphur            Manganese      Iron

a)            3.10                 0.98                 0.10                 0.06                 0.40                 95.0

b)            0.04                 0.02                 0.10                 0.06                 0.10                 99.6

c)            0.45                 0.04                 0.10                 0.06                 1.15                 98.0

When the carbon content is about 0.9%, a steel is formed that is tough and ductile and which can be cheaply shaped by mechanical working.