Page 178 - CW E-Magazine (24-6-2025)

Special Report

heavily pigmented primers such as in-
organic zinc silicates or water-based
coatings, although can occur with other
over-thick systems also. In this coating
failure the dried paint film has an
appearance of a dried-out mud flat. The
cracks appear as a network that can
vary in size and amount as illustrated
in Figure-27.
Fig. 23: Grimming Fig. 25: Impact damage
coating material, often seen where Prevention methods Prevention methods
dark colours are over coated with light Prevent the material from impact Apply only the recommended coating
colours as illustrated in Figure-23. damage. thickness. Use recommended application
techniques with suitably formulated
Prevention methods Intercoat contamination products.
Apply adequate dry-film thickness The contamination could be pre-
between individual coats. Use opaque sent due to inadequate washing down, orange peeling
coatings with good opacity. Described as failure of the paint
film to flow out, it is usually caused
Heat damage
It is the effect of high temperature
often applied to the reverse side of a
steel plate from burning, welding, or
fire. Figure-24 illustrates heat damage

Fig. 26: Intercoat contamination
salts from weathering of shop primer, Fig. 28: orange peeling
or deposits from nearby operations.
In this defect cracks radiate from a by poor application techniques and/
point of contamination as shown in or using incorrect solvent blend. This
Figure-26. coating failure gives a uniform pock-
marked appearance, particularly in a
Fig. 24: Heat damage Prevention methods spray-coating, in which the surface of
of a coating showing discoloration, Carefully inspect and test the sur- the paint film resembles the skin of an
detachment and/or blistering and face before paint application and wash orange (Figure-28).
general degradation of a paint. down with fresh water if required.
Prevention methods
Prevention methods Mud cracking Use correct application techniques
Ensure that all welding/burning is Caused due to over-application of with suitably formulated products.
completed prior to painting.
Peeling
Impact damage Peeling is the reduction in bond
Impact damage in a coating leads to strength of the paint film due to contami-
a relatively brittle coating and is often nation or incompatibility of coats. It is
seen on glass-fibre-reinforced plastics. similar to flaking and tends to be associ-
Also occurs when steel is deformed ated with soft pliable fresh coatings that
by impact. Figure-25 shows impact can be pulled away from or flake away
damage of a coating where cracks are from the substrate or from between coat-
radiating from a point of impact. Fig. 27: Mud cracking ings due to loss of adhesion (Figure-29).

178 Chemical Weekly June 24, 2025

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