Page 175 - CW E-Magazine (24-6-2025)

Special Report

paint giving undesirable discoloration Bubbles or Bubbling
or staining. Bubbles can form if trapped air
or solvent within the coating is not
Prevention methods released before the surface dries or
Use correct coating specification there is air entrainment during mixing.
and materials. Use compatible materials Bubbles can also form if high ambient
and use appropriate sealer coat if temperature is maintained during appli-
possible. cation. It can be found with factory-
applied coatings where application is
Blistering Fig. 9: Bloom (Blush) done by dipping, electro-deposition, or
Many mechanisms can be involved, blooming. A typical bloom/blush roller coating. Bubbles within a paint
including osmotic gradients associated defect showing hazy deposit on the
with soluble salts, soluble pigments, surface of the paint film resembling
corrosion products, retained solvents, the bloom on a grape, resulting in
and solvents from cargoes. Non-osmotic loss of gloss & a dulling of colour, is
blistering is associated with cathodic shown in Figure-9.
disbonding, thermal gradients related
to cold-wall effects, and compressive Prevention methods
Apply and cure coating systems
under correct environmental conditions
and follow the manufacturer’s recom- Fig. 11: Bubbling
mendations.
film appear as small blisters. These may
Bridging be intact or broken (leaving a crater) as
This type of coating failure is due shown in Figure-11. It can be found in
to poor application such as applica- excessively thick paint films, especially
if spray applied, and with roller appli-
cation and should not be confused with
Fig. 8: Blistering
blistering.
stress. Dome-shaped projections or
blisters in the dry paint film through Prevention methods
local loss of adhesion & lifting of the In spray application, adjust visco-
film from the underlying surface is sity with thinners and follow data sheet
shown in Figure-8. Blisters may contain requirements for maximum application
liquid, vapor, gas, or crystals. temperature. Use correct mixing equip-
ment to ensure air is not stirred in
Prevention methods Fig. 10: Bridging during mixing. A de-foaming agent can
Ensure correct surface prepara- tion of high viscosity paint system and be added to emulsion paints for prevent-
tion and application. Apply a suitable failure to brush paint into corners and ing bubbling.
coating system after testing for soluble over the welds, etc. The covering over
salts. Consider the possibility of the of unfilled gaps such as cracks or cor- cathodic disbonding
different blister mechanisms in the ners with a film of coating material It can take place when high overvolt-
particular environment. introducing a weakness in the paint film, age exists, principally from impressed
leading to cracking or flaking off, is current cathodic protection (CP) sys-
Bloom (Blush) illustrated in Figure-10. tems but also can take place to a lesser
Paint film exposed to condensation extent with sacrificial anodes when the
or moisture during curing, especially Prevention methods number of anodes is excessive. Factors
at low temperature (common with Brush-apply a stripe coat into cor- contributing to cathodic disbonding are
amine-cured epoxies) are subjected ners and welds and fill all cracks or incorrect installation, poor monitoring,
to ‘Bloom or Blush’ failure. Incorrect weld them prior to application of the and incompatible coating systems.
solvent blend can also contribute to full coating system. Figure-12 shows blistering & delami-

Chemical Weekly June 24, 2025 175

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