Page 179 - CW E-Magazine (24-6-2025)

Special Report

Fig. 29: Peeling Fig. 31: Rippled coating Fig. 33: Rust Rashing
Prevention methods caused by poor application techniques surface profile. Fine spots of rust
Use correct coating system and (Figure-31). appear on a paint film, often when a
specification applied to clean and un- thin primer coat is applied. The initial
contaminated surfaces. Prevention methods spots rapidly spread over the surface,
Do not apply paint under unfavour- resulting in a film of rust through which
Pinholes able conditions. Use correct application the individual spots are difficult to dis-
It is caused due to solvent or air equipment and ensure proper work- cern (Figure-33).
entrapment within a paint film. It is a manship.
Prevention methods
Runs Ensure that an adequate thickness
Over-application of paint, exces- of a primer coat is applied to cover
sive use of thinners, incorrect (or lack the surface profile, and check that the
of) curing agent, or poor workmanship surface profile is not too large.
can cause this defect. It invariably
Rust spotting
Low film thickness, voids and holi-
days, but also defects in the steel, such
as laminations and inclusions can cause
Fig. 30: Pinholes
this defect. Too high a surface profile
common problem when coating porous may cause penetration of peaks through
substrate such as zinc-filled primers, a paint film and cause rust spotting. It
zinc silicates, and metal-sprayed coat-
ings. Pinholes can also be caused by
incorrect spray application or incorrect
solvent blend (Figure-30). Fig. 32: Runs
involves narrow downward movement
Prevention methods of a coat of paint, often apparent from
Use correct application techniques accumulation of excessive quantities
with suitably formulated products of paint at irregularities such as cracks
with correct solvent blends and envi- and holes where the paint continues to
ronmental conditions. Check spray- flow after the surrounding surface has
equipment and distance of spray gun set (Figure-32). Fig. 34: Rust spotting
from the surface. Applying a mist coat may also occur from metallic contami-
can be helpful. Prevention methods nation of a coated surface by grinding
Use correct application techniques dust. Individual spots of rust appear
Rippled coating and apply recommended dry-film on a paint film and frequently start as
Strong wind blowing across the thickness. localized spotting rapidly increasing in
surface of wet paint causes it to ripple. density as shown in Figure-34.
Where this occurs on the underside, Rust rashing
the ripples can hang down in the form It is caused due to low film thick- Prevention methods
of small stalactites. It can also be ness, often in combination with a high Ensure that an adequate thickness

Chemical Weekly June 24, 2025 179

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