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28 POWDER COATING, August 2017 —Stage six: Rinse — S t a g e s e v e n : D I P i n o r g a n i c zirconium I hope this helps you understand the issues, tradeoffs, and limited potential for being successful. Getting good weather resistance PROBLEM I'm currently doing some testing with triglycidyl isocyanurate based (TGIC-based) polyester and a thermoplastic on steel. We have had many problems in the past with our TGIC finish not holding up past 2 or 3 years when applied to steel and left out in Wisconsin weather. I have a plate of steel blasted white with the back untreated. The front was treated with two different products (half each), coated with TGIC-based polyester, scratched to metal, and set to soak in a heavy brine. Any advice? S.J., Ettrick, Wis. SOLUTION For the TGIC outdoors, if you want it bulletproof, first apply a zinc-rich epoxy powder primer, par- tially cure, apply the TGIC polyester powder coating, and fully cure. This coating system is good for 3,000 hours ASTM B-117 salt-spray resistance. PC Editor's note For further reading, see articles under the Surface preparation heading in the "Index to Articles and Authors 1990- 2016" Reference and Buyer's Resource Issue, Powder Coating, vol. 27, no. 6 (December 2016), or search the Article Index at www.pcoating.com. All articles listed in the archive are available for free download to registered users. rectly. The acid source must be neu- tralized before the phos deposition. This requires a rinse and a neutralizing stage where you bring the pH of the substrate back to 8.0+. Soda ash, bo- rate, or a mild alkaline salt is necessary to neutralize the remaining acid salts. Of the five to six systems I've person- ally seen, the acid bath concentration is very tenuous; it's a very small window of concentration to effectively remove the scale without over-activation. Most of the time, the bath is too weak, which provides incomplete removal, or it's too strong and softens existing scale if hot rolled steel (HRS) is also being run. So, even if you do it correctly, the cost is usually too high, the results inconsis- tent, and the phosphate poor. This leads to pinholing film interruptions and poorer salt spray. Platers under- stand this and have the necessary addi- tional rinses and tanks to control the process. Powder coaters try to slam it into five stages—and it doesn't work. The other way to solve this problem (on lighter-gauge materials) is to switch from oxygen to nitrogen when cutting the metal, slow the feed rate down, and eliminate the oxide formation. Everyone says that it's too costly to do mechanical or oxygen elimination correctly. The situation you have sounds pretty iffy. By taking the alkaline cleaner out of stage one, you open yourself up to re- maining organic soil issues. Second, most acid systems are poor wetters to begin with. If I understand you cor- rectly, the main problem is solution (acid bleed-out). If the acid is entrapped and bleeds out during heat up (dry off or cure), it's not solvable in a spray sys- tem. It doesn't matter if it's entrapped hydroxide or acid; both will interfere with melt, flow, and cross-linking. The only possible solution would be to have a spray immersion combination. Depending on the size and ID of the entrapment, you may never get the so- lution adequately neutralized and ef- fectively rinsed out anyhow. My guess is that you'll be throwing good money after bad by thinking two extra stages will solve it. The last example I can provide is based on my experience with a customer who beat this issue by doing the following: 1. They buy really high quality HRS (no flaking, partially adherent scale, rust, and so on). 2. They mechanical wire brush edge scale off of HPO (class A surfaces) and ignore scale on underbody class C areas. 3. They pay close attention to both chemical and mechanical aspects of the washer. Now I'll give you what you may want to try based on the information you've provided. Please understand that I haven't been to your plant, so the rec- ommendations to test for here come with no promises: Option 1 —Stage one: Alkaline clean —Stage two: City overflowing rinse (good GPM); counter-flow this rinse to stage four, retain some of the rinsed alkalinity to provide some alkaline neutralization. —Stage three: Acid (sulfuric is too strong; go to a proprietary blend or citric-enhanced) —Stage four: City overflowing rinse — S t a g e f i v e : M i l d a l k a l i n e s a l t neutralize (pH 8.0-8.5). Test! —Stage six: Rinse —Stage seven: Phosphate —Stage eight: Rinse —Stage nine: Seal or reverse osmosis water Option 2 —Stage one: Alkaline clean —Stage two: City rinse —Stage three: Acid —Stage four: Rinse —Stage five: Neutralize