Page 180 - CW E-Magazine (24-6-2025)

Special Report

of a primer coat is applied to cover the problem. Figure-36 shows sag caused
surface profile. Use a thicker coating by the downward movement of a coat
system or a lower blast profile. Protect of paint that appears soon after applica-
coating from contamination with grind- tion and before setting, resulting in an
ing dust and so on. uneven area with a thick lower edge.
They are usually apparent on local
Rust staining areas of a vertical surface and, in severe
It can be caused due to water runoff situations, may be described as
from a rusty surface above a soundly curtains.
coated surface. Rust staining occurs Fig. 38: Skinning
when the rust is wetted-out and con- Prevention methods of a layer of skin on the paint surface
Use correct application techniques in a container.
with suitably formulated products.
Prevention methods
Settlement Use airtight container and store
It can be caused due to old stock, according to data sheet.
heavily pigmented paint, and incorrect
formulation of product. It is usually a Solvent lifting
It happens when incompatible paint
systems are used. Topcoats with a
strong solvent blend can react with
Fig. 35: Rust staining
taminated water runs over and discol-
ours other items or locations. It is usu-
ally more of an eyesore than a defect.
The coating itself may not be defective,
only stained. Figure-35 shows light
rust staining on the surface of the paint
caused by the precipitation of ferrous Fig. 37: Settlement
oxide from adjacent exposed steel. problem with zinc-rich primers. The
sedimentation of the solid constituents
Prevention methods comprising pigments and extenders Fig. 39: Solvent lifting
Ensure adequate design and suitable from the binder and solvent while previous and weaker solvent-blended
maintenance. standing in a container is shown in coatings causing this defect. Eruption of
Figure-37. Settlement that occurs after the surface of the paint film, wrinkling
Sags mixing and during application can and blistering leads to a weak surface
It may be caused due to over-appli- result in different shades and perfor- and ultimately causes coating breakdown
cation of paint, excessive use of thin- mance in different areas. as shown in Figure-39.
ners, incorrect (or lack of) curing agent,
or poor workmanship. In extreme cir- Prevention methods Prevention methods
cumstances, it could be a formulation Use products within shelf-life. Use Use correct coating specification,
adequate mixing procedures. Keep over-coating times, and materials. Con-
paint mixed or recirculated during duct compatibility trials with under-
spray application. coat/topcoats.

Skinning Solvent popping
It is caused by absence of anti- Incorrect solvent blends, porous
skinning agent, use of non-airtight surfaces, incorrect environmental con-
container, and hot storage conditions. ditions, or high surface temperatures
It often occurs when partly-used cans can cause this defect. Solvent (clear)
Fig. 36: Sags are used. Figure-38 shows formation bubbles on the surface of the paint film

180 Chemical Weekly June 24, 2025

Contents Index to Advertisers Index to Products Advertised