Metal Finishing Guide Book

2012-2013

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8. Areas with defects were sanded smooth and feathered 9.Additional coat of polyurethane was applied 10. Finally, glazing, adhesive labels, etc. were applied The experimental railcar was then put into service for six years, during which it traveled many times through the desert, in tropical environments, along the coast, etc. The railcar experienced a wide range of environmental conditions. Nevertheless, sufficient solvent still remained in the coating to cause catastrophic delamination! Figure 4. Environmental contaminants, such as moisture, can gradually diffuse through the damaged coating and collect in the voids. This can build up a pressure inside the voids and accelerate the delamination failure. CASE #2: A large multi-ton overhead crane had been fabricated for use in a steel mill. The crane had been freshly painted with an alkyd primer followed by three coats of a solvent-based acrylic-modified alkyd topcoat. Each coat had been applied weton-wet. Unbeknownst to the manufacturing manager, solvents in alkyds and modified alkyds of 1970s vintage evaporate slowly. At around 4p.m. on Friday afternoon the freshly coated crane was carefully lifted onto a special heavy-load transporter ready for a 300 mile drive to the steel mill. A few minutes before the truck driver was about to leave, the manufacturing manager quickly performed an adhesion test. To his horror he discovered that the entire paint system lifted off. Frantically, he called Ron Joseph, whose employer had supplied the paint. After some questioning and subsequent adhesion checks it was revealed that adhesion was good on one side of the crane but was catastrophically poor on the other. The ���good��� side had been exposed to the sun for the past week, while the ���poor��� side had been in the shade. It was now too late for the manufacturing manager to take the crane off the transporter; therefore, Joseph recommended the crane be positioned so that the ���poor��� adhesion side would get the maximum amount of sunlight during its journey to the steel mill. Hopefully, by the time the site inspector would perform his adhesion check sometime during the following week, solvents would have evaporated and the coating would pass the adhesion test. As expected, the crane passed the test. CONCLUSIONS: ��� It is poor practice to apply too many (or excessively thick) coats of paint without allowing the solvents to properly evaporate between coats. ��� Beware of applying coats wet-on-wet, unless specifically recommended by the manufacturer. ��� Thick coatings are not conducive to rapid solvent evaporation. 579

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