| Q: We are building a power plant in which the specification calls for metal cladding coated with epoxy primer (5 microns) and acrylic enamel top coat (20 microns), while the products available locally are coated with primer (5 microns) and polyester top coat (20 microns). Can you please advice about the difference in specification and field of usage between the acrylic enamel and the polyester? What is more suitable for outdoor usage and direct sunlight exposure? A: The specified film thicknesses of 5 microns for the primer and 20 microns for the top coat suggest that the customer requires you to purchase coil coated metal cladding. I believe you will be better off with the acrylic top coat on the exterior surface, (facing the sunlight), and the polyester on the inside, (internal surface). If both surfaces are coated with the same resin, I would suggest acrylic on the top and bottom. While acrylic coil coatings are reputed to give you long life, paint manufacturers will usually warrantee PVDF coil coatings for 20 years. PVDF is polyvinylidene (di)fluoride and is known to be superior to acrylic. It is also probably more expensive, but I don’t know what the cost difference is. You can get all the details for this project by going to the National Coil Coatings Association or the European Coil Coatings Association websites. Both web sites have excellent literature on this topic. PAINT FAILURE OF COIL-COATED ROOF Q: We have a large roof over our warehouse made of coil coated galvanized steel. The roof was installed approximately 12 years ago and is now starting to show signs of rust and peeling. I understand that many roof manufacturers offer warrantees of 20 years and more for coil coated sheets. What would I need to do to determine if I should challenge the warrantee? Do I need to retain the services of a paint consultant or NACE certified coatings inspector? A: Four predominant resin systems are used for coil coating galvanized steel; acrylic, polyester, polyvinylfluoride (PVF) and polyvinylidene(di)fluoride (PVDF). The resin system that usually carries a long-term warranty is the last one, PVDF. The other three resin systems are more common and less expensive, and are not expected to last quite as long on under exterior exposure conditions. Before you can consider a claim, you will first need to determine if the roof came with a warrantee. If so, you would likely retain the services of a coating consultant or a well qualified NACE certified coating inspector. As a full time paint consultant and expert witness, I would first want to inspect the roof and take photographs to see if there are any trends. For instance, do the paint defects predominantly appear of one side of the roof (north, south, east or west)? Is the nature of the paint failure the same, or are there different failure modes, such as the entire paint system peeling from the galvanizing; top coat peeling from the primer; corrosion of the galvanizing itself, and other possibilities? Are the failures near the top of the roof, or towards the gutters? Have the gutters been cleaned so that rain can run out easily, or are they filled with leaves and debris allowing rain water to stand for days or weeks and affect the lower edge of the roof? Is there an unusual amount of mold on the coil coating? While on site, I would collect small samples of the failed coating as well as samples of the coil coating together with the galvanizing substrate. Once back in my laboratory, I would perform an analysis using Fourier Transform Infrared Spectroscopy (FTIR). This is an analytical technique that can identify the resin type and confirm if it is a PVF, PVDF, acrylic or polyester. I would want to examine the coating for microcracks, especially along the profiles (or sharp bends) on the metal, and also between the large profiles. A strong magnifying glass can be used on site, but generally I would make a cross-section of the sample and examine it under a stereo microscope or a Scanning Electron Microscope (SEM). This will identify if the coil coating was applied to clean or corroded galvanized steel and it might also tell me more about the coil coating itself. More importantly, I would be able to determine how many layers of paint were applied and the thickness of each layer. Film thickness is a critical parameter for long term performance. Is the failed coating brittle or flexible? Can I determine if the galvanizing was properly pretreated before the primer was applied? During a coating inspection of the coil-coated roof, I would look for sources of unusual contamination that might have caused the “premature” degradation. Perhaps your roof is near a chemical plant, or perhaps there are other aggressive environmental pollutants in your local area that might have contributed to the coil coating failure. If you are in close proximity to the ocean then all bets are off because marine salts from ocean fog can significantly shorten the life of the roof coating. Different paint specifications are required for roofs that are exposed to aggressive environments. A thorough coating inspection of the roof and an analysis of a sample of the coil coating should be able to tell you if the coil-coated roof is defective. Ron Joseph is a paint and coating consultant employed by Exponent Failure Analysis, Menlo Park, CA. (408) 507-7927. | 
