Page 173 - CW E-Magazine (24-6-2025)

Special Report

Alligatoring may be caused by a Undercutting occurs when corro-
coating that does not have enough flexi- sion forms on a metal substrate beneath
bility when the temperature changes, a coating. The corrosion products lift
by applying the coating with a greater the protective coating up and away
than recommended thickness, by using from the substrate, allowing moisture
an inflexible hard top-coat over a more and salts to reach more areas of the
flexible soft undercoat, or by not allow- substrate, thus facilitating more corro-
ing sufficient time for the undercoat to sion as it creeps along the surface to
dry before applying a subsequent coat dislodge the coating. Figure-4(a) shows
or top-coat. It can also be caused by Fig. 3(b): crazing undercutting caused by corrosion pro-
heating the finished piece (e.g., in an defect. Figure 3(b) shows another ducts forming on a metal pipe. Visual
oven) to accelerate drying. picture of crazing of coating similar to corrosion beneath a paint film, often
checking but cracks are generally wider
Prevention methods and penetrate deeper into the film.
Before starting, ensure all the coat-
ings in a multi-layer system are com- Crazing is common with acrylic
patible. Select a coating that is more paints that develop a skin when dry-
flexible or one containing reinforcing ing while the portion of the coating
fibres or pigments. Follow the manu- closer to the substrate is still liquid. It is
facturer’s guidelines and perform the principally caused when application
coating application when the ambient temperature is kept too low; there is in-
temperature is steady and not too hot. compatibility with previous coating, aging Fig. 4(a): undercutting
Avoid extreme temperature variations. affect, or when film thickness is high.
Do not permit excessive film thickness called creep, travels beneath the paint
during the coating process, and allow Prevention methods film and lifts the paint from the sub-
the primer or basecoat to dry completely First, ensure all the coatings in a
before applying the top-coat. multi-layer system are compatible.
Follow the manufacturer’s recommen-
crazing dations regarding temperature. If out-
Crazing presents itself as a network doors, perform the work only when the
of wide cracks on the coating’s surface drying conditions are appropriate for
and may or may not penetrate to the the coating. Avoid extreme atmospheric
substrate. In a general sense, crazing is variations or cold temperatures.
caused by tension within the coating.
Specific causes include attempting to Apply the coating thinly and avoid
apply the coating in cold ambient tem- thicker sections or puddles. Several Fig. 4(b): undercutting
perature conditions, applying two dif- thinner coats are better than one thick strate. Severe cases can show as blister-
ferent coating products that are incom- coat. Principally, it can be prevented by ing, flaking, cracks, and exposed rust as
patible, or by applying the coating with applying a thinner coat of paint, adding shown in Figure-4(b).
an excessive film thickness. Figure-3(a) slower-drying solvent, checking that
indicates a typical crazing coating application and drying conditions are Undercutting is caused by apply-
correct for the paint system used, and ing a coating to a surface that is already
checking compatibility. corroded without proper surface prepa-
ration. The root cause of the insuffi-
undercutting cient surface preparation or inadequate/
Undercutting appears as a coating improper coating application is often
that flakes or peels off around corroded because the location is hard to reach,
portions of the substrate. At this point the which makes inspection and remedia-
asset is in jeopardy because not only is tion difficult.
there a coating problem, but there is also
Fig. 3(a): crazing coating defect a (previously hidden) corrosion problem. Undercutting is common when a

Chemical Weekly June 24, 2025 173

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