7.4 Types of Failures

(C) Cratering

Identification:

Small, uniform indentations are formed in the film The indentations are like pits.

Description:

The defect is generally caused by mixing of contamination and dirt with the wet film. Some binders have inherent cratering tendency. On encountering contamination, they form craters. Cratering occurs before the complete drying up of the film. Cratering areas are potential sites for coating failures.

Remedy:

• Remove the wet coating and re-paint. After the coating has cured, the coating surface in the crater area must be removed by sand blasting and applying another coat on the cratered area.

(D) Dry Spray

Identification:

The surface does not form a continuous film and it looks rough, flat and gritty. 

Description:

This problem occurs when coatings are applied by spray. It is not of much concern in slow drying coatings based on oil materials, but lacquer type coatings such as chlorinated rubber and vinyls are sensitive to overspraying. The atomized particles coming out of the spray gun are not wet enough to flow and they fall outside the spray pattern. The particles do not find sufficient time to level because of the fast rate of evaporation.

Remedy:

• Before curing, remove the affected area by dry brushing followed by solvent wiping. After curing, sand blast and apply another coat.