Products Finishing

JAN 2013

Products Finishing magazine is the No. 1 industrial finishing publication in the world. We keep our readers informed about the latest news and trends in plating, painting, powder coating, anodizing, electrocoating, parts cleaning, and pretreatment.

Issue link: https://pf.epubxp.com/i/100758

Contents of this Issue

Navigation

Page 49 of 71

p aintin gCL IN IC pfonline.com/experts n Film Thickness measuremenT Q. I am in charge of quality control and check dry paint film thickness on the steel tubes and pipes we produce before they leave the plant. In many cases, the pipes are butt welded together to form long runs. In other cases, flanges are welded to the pipe ends. We have just started producing a line of pipes made of stainless steel, and I have encountered a strange thing while checking the dry film thickness on them. The film thickness gage will read the coating thickness on the weld joints between two pipes and the flanges but not on the pipe itself. Can you explain this? O.F. a. Obviously you are using a magnetic dry flm thickness gage. Certain grades of stainless steel are nonmagnetic and therefore would not give you a reading using a magnetic gage. However, when the stainless steel tubes and pipes are welded, the high temperature can cause a change in the chemical composition of the metal, making it magnetic and enabling you to get a reading in those areas. n PainTing hoT-DiPPeD galvanizeD sTeel Q. I work for a steel fabricator, and we recently received a request to bid on an outdoor structure made from hot-dipped galvanized steel, which we have not worked with before. The customer���s specifications require the structure be painted with a two-part epoxy primer, and then he will apply his own topcoats in various colors. I understand there are a number of cleaning methods and pretreatments available for preparing hot-dipped galvanized steel, but my sources do not recommend these with confidence. Do you know of a successful and fool-proof method? P.P. a. If I were you, I would turn to other sources for painting advice. Your prospective customer knows that a fnish system consisting of a two-part epoxy primer topcoated using a weatherresistant enamel will provide outstanding corrosion resistance to his steel struc- tures. The problem with painting galvanized steel products, including hot-dipped, is the waxy or oily material applied at the mill. In outdoor and some indoorexposure conditions, galvanized steel will form a loose coating of zinc oxide on its surface. The waxy or oily material is applied at the mill to prevent this ���white rusting,��� but it forms a barrier coating that must be removed before painting. Pretreatment procedures to remove these waxy or oily materials would include use of aqueous cleaners, solvent degreasers and phosphate coatings. Zinc phosphates used as pretreatments for galvanized steel prior to painting will require equipment you may not have in your plant. In that case, you can remove the waxy or oily material by simply applying a hot aqueous solution of an alkaline or detergent cleaner to the hotdipped galvanized surfaces. They can be applied using a steam spray cleaner or power wash sprayer. n abrasion-resisTanT coaTing Q. We refinish metal cannery machine parts. The paint must be abrasion-resistant and withstand daily high-pressure washings of 1,000 psi. Where the coating has been damaged, there is often severe rusting. We then remove the rust and all the old paint on the parts by sandblasting to bare metal. We apply a wash primer and then a twopart epoxy topcoat. Is this the best method? M.J. a. In general, I like your approach to the refnishing problem: sandblast, wash primer and topcoat using a two-part epoxy enamel. However, I suggest you modify your fnish system by replacing the wash primer with a two-part epoxy primer because of its superior corrosion resistance. I also suggest you replace the epoxy enamel with a two-part polyurethane because of its superior abrasion resistance. My recommended fnish system is: 1) Sandblast to white metal in accordance with SSPCSP5 White Metal Blast Cleaning. 2) Prime with a two-part epoxy primer at a dry flm thickness of 1 mil. 3) Topcoat with a two-part polyurethane enamel at a dry flm thickness of 2 mils. n sPray gun Flow raTe Problem Q. We have problems achieving consistent paint flow through our spray guns (typical rates range from 10 to 50 g/min). We spray small round parts up to 50 mm in diameter by mounting the parts on spindles and moving them past the spray guns using a chain-on-edge conveyor system, producing about 3-5k parts per hour. The paint is supplied pre-mixed in 2-L tins, and we pump this using a regulated diaphragm pump set at 60 CARL IZZO, Consultant, paintingclinic@pfonline.com 48 JANUARY 2013

Articles in this issue

Links on this page

Archives of this issue

view archives of Products Finishing - JAN 2013