However, after applying the clearcoat powder at 2.5 to 3.0 dry film mils and then curing at 400ºF for 12 minutes, we see various shades of amber. We tried underbaking, thinking the powder was overbaked, but found that, although the finish looked closer to the natural look of the substrate, the gloss was 25 degrees too high. Does the chemistry in the iron phosphate need to be adjusted differently for clear coatings, or is this a direct result of the iron phosphate process? We asked many sources. Some suggested using an alkaline cleaner and rinse, followed by a nonchrome rinse and not using iron phosphate at all. Would this be a problem if the end product will be used in a store fixture in a somewhat controlled environment? Can you suggest a process that would give us some consistency with this project?
You may be correct regarding the work-hardening factor, but I also suspect the difference in color comes from the slight difference in steel alloy composition between the sheet and tube stock. In addition, the variance in phosphate spray impingement pressure can produce color differences. I doubt if any phosphate formulation can consistently give uniform color coatings because phosphate-solution concentrations, temperatures and drainage factors all affect color. Talk with the chemical supplier about this, but I don't hold out much hope for you to be able to avoid color discrepancies.