Q: Is there any indication that static electricity could be causing defects such as fisheyes, cratering, or bits in the powder coating process? Is any static electricity generated in any one of the powder coating systems? Also, what products are commonly used when repairing the types of defects noted above?
A: Wow, you have a lot of questions. Let me break them down.
Yes, there is static electricity and from more than one source when dealing with powder coating application. As you probably already know, powder coatings are electrostatically charged and typically by a corona mechanism near the tip of the business side (outlet) of the spray gun.
This works by very high voltage (80,000-100,000 volts) being delivered to the electrode. This high voltage ionizes the air, creating a field of electrons or negative charge. (Don’t worry, the current is very small – typically micro-amps – so the process is relatively safe.)
The powder is pneumatically conveyed into this electrostatic field, picks up a charge, then heads to the nearest ground. The object of the game is to make your parts the closest ground. The powder then deposits on the surface of said part.
It’s important to note that static electricity is also generated elsewhere in the finishing system. The mere fact that powder particles are being conveyed into fluidizing hoppers, through powder pumps, hoses, and spray guns creates static charges. This extraneous generation of charge affects powder deposition and can influence transfer efficiency.
Does static electricity cause fisheyes, cratering, and bits? Probably not. Very characteristic defects are caused by electrostatics, however.
Excessive charge can build up in a powder coating layer if the powder is applied too thick, too quickly. This phenomenon is known as back ionization. The buildup of charge causes powder particles to microscopically burst away from the substrate. This causes micro-defects that resemble volcanoes. These defects cause a rumpled appearance that looks more like localized macro orange peel, for lack of a better description.
Fisheyes are caused by a different mechanism and are more distinct in their appearance. Fisheyes are created by a significant differential in surface tension between a molten powder film and a contaminant. Common contaminants are lubricants and oils. Silicone lubes and penetrating oil (e.g., WD-40) are some of the worst actors.
Fisheyes are gross defects characterized as deep circular voids in the coating film that reach the substrate. Craters are ambiguously defined as defects similar to fisheyes but smaller in diameter. Craters often do not reach the surface of the substrate and typically look like dimples.
As for bits, I imagine you are referring to unmelted protrusions in the finish of the cured powder coating. These are not caused by static electricity. Common sources of bits are environmental dirt (oven, spray area, unclean application equipment, etc.), unclean substrate, or dirty powder coating. They are typically more prominent at thin films (e.g., < 1.5 mils).
If the preponderance of bits decreases with thicker coating films, then you can suspect either the powder or a contaminated substrate surface. If film thickness is not a factor, then the contaminant is probably environmental and is deposited on the powder after it has been applied.
Regarding repair, fisheyes, craters, and bits need to be buffed with an abrasive (Scotchbrite™ or 200-grit sandpaper), the surface wiped (e.g., with acetone), dried, and recoated. Alternately, you can consider using a liquid paint based touch-up if the repair is small and localized. Be careful, however, as the touch-up may not meet the performance of the original powder coating finish.