Case 1

Cause of Defects

The chief cause of bittiness is attributed to the

• presence of impurities
• application of paint using uncleaned brush
  • If the brush is not cleaned properly after painting and is reused, certain amount of dry pigments may be left in the bristles. When painting, particles are brushed onto the paint and bittiness occur.
• presence of pieces of broken up skin in a paint are a further potent cause of bittiness
  • Skinning of paint takes place during paint storage and this skin is not thoroughly removed before stirring. If this paint is applied, bittiness of paint occurs where small particles are left on the paint (Figure 1a to c).

Mechanism:

Pigment particles are attracted to the active interface between the surface of the impurity and the medium, within a few seconds an aggregation of pigment particles surrounds the original microscopic fragment, forming a projecting lump many times the size of the original fragment and two or three times the thickness of the paint film. This causes obvious defects in paint film.