“I have just had sheets of 9″ by 48″ flat perforated steel, 16 gauge with 5/16″ diameter holes powder coated by a local company. My intent was to get a high dielectric strength using Corvel 1700 epoxy series powder that gives 800 to 1000 volts per mil. The problem was the 5/16″ holes did not get a good coating on the inner part or around the edge of the hole which causes an electrical breakdown at the high voltage (2000 to 5000 volts) and low current needed for the application . The build seems to be about 5 mills on the flat surface. How many mils can be put on with normal coating? Can the sheets be coated again without stripping off the first coat to get a film build of 10 to 15mils or more? Any suggestion on how to proceed would be very helpful. Thank You.”
“Most powder coatings can be applied using standard electrostatic spray techniques up to 10 mils. At that point the powder coating will be “self limiting” because the part ground is no longer sensed by the charged powder particles. Preheating the part has been an effective way to overcome this problem in high film build applications.
The sheets can be recoated without stripping the previous coats if some precautions are taken. First the surface must be clean. Second the part must be adequately grounded (less than one megohm resistance). Finally the total number of coats cannot exceed two if the part is fully cured between coats without risking coating degradation from overbaking.
The problems you are having with the holes is called edge coverage. This coating characteristic is formulated into the powder coating and controls the flow and build of the powder on all edges. This combined with the Faraday problem in 5/16″ diameter holes will prevent full coverage in this critical area. I suggest that you check the coating spec sheet to ensure that the coating has the edge coverage characteristics you need and preheat the part prior to coating to overcome the Faraday problems.”