Coating Defects-Common Coating Failures and Causes
Non-Drying Film (Failure to Cure)
Non-drying film is a common problem on many projects, frequently caused simply by not adding the cure to the base, adding the wrong cure, or not adding the correct amount of cure during mixing. Other issues that might cause non-drying or curing of a coating film are: problem with the coating material sent from the manufacturer, environmental issues, wrong thinner or contaminated thinner; moisture in some generic thinners can react with the cure
Blushing (Amine Sweating)
If cured during conditions of cool ambient temperatures, dropping temperatures, or high humidity, amine-cured epoxy resin coatings can develop a surface oiliness or exudates, commonly referred to as “amine blush” or “sweating” .This is caused by the absorption of carbon dioxide and water into the coating film and its reaction with the amine curing agent. Some of the problems can be: surface tackiness or greasiness, incomplete cure, poor adhesion, poor adhesion on over-coating, coating discoloration over time, poor gloss retention.
Runs, sags, curtains, or wrinkles may be caused or worsened by: applying the coating too thickly, using too much thinner or using the wrong thinner, surface too hot to apply the coating, applying a coating at the end of its pot life, wrong thixotrope used in manufacturing , improper spray technique
Discontinuities, Skips, Holidays, and Missed Areas
These defects are exposed areas of the substrate or previous coating, caused by poor application technique, lack of stripe coating, and/or lack of or poor inspection. Quick recognition and repair means the repair is easier and it will blend in better.
Chalking is a powdery, friable layer on the surface of a coating normally caused by exposure to UV light. It can also be due to exposure to other forms of radiation including nuclear radiation. The UV light or radiation breaks down the bond between molecules in the coating film. It is most common in epoxy coatings but can be seen in almost all coatings left exposed to causative conditions for a long enough period of time.
Cratering is the formation of small bowl shaped depressions in the coating caused by air trapped in the coating. The air forms a bubble that bursts, leaving the crater. Cratering is common in coatings that are roller- or brush-applied by an inexperienced applicator. Improper mixing procedures can also trap air in the coating, thus causing cratering.
Air Voids (Vacuoles)
Initially invisible, these pockets of air trapped inside the coating film generally lead to failure in the near future. They are caused by air trapped in the coating during mixing. This typically occurs when the mixer is run too fast. This creates a foam or bubbly surface on the liquid coating.
Pinholes are very small holes in a coating, typically caused by painting over an inorganic zinc coating (IOZ) or sprayed metal coatings. Air or solvent, escaping through a porous film, cause these tiny holes. Zinc coatings set up so quickly that the small holes will not fill back in. This defect is easy to avoid by simply using a mist coat followed by a full coat. Repairs for cratering, vacuoles and pinholing are very difficult since the hole generally just re-forms in any coating applied over them.
Rust stains are normally not a problem except as a matter of aesthetics. If necessary, clean by sanding and over-coating. The only way to stop this from occurring is to repair or replace the item that is rusting and bleeding onto the coating.
The defect where visible distinct boundaries of wet and dry layers of paint had overlapped producing lines of color density or gloss difference is called lapping. Lapping occurs in fast drying paints where the solvents evaporate quickly leaving short wet edge time or if the painted surface (steel in direct sun) is too hot.
Uneven film formation with hills and valleys in paint resembling the skin of an orange. The defect can be caused by improper application technique where the spray gun is too close to the surface or the air pressure is too low for proper atomization of the paint. Hot air temperatures may also cause this defect because paint droplets dry before they can flow out and level smoothly together.
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