With powder coating comes pinhole problems. Pinholes are one of the most common and annoying finish defects to deal with. The good news is that pinholes are preventable if you understand what causes them. It’s also sometimes possible to salvage parts that have pinholes in their finish. There are many causes for pinholes, so we’ll start by listing the process changes that will help you eliminate, or at least minimize, your pinhole issues. We’ll conclude by explaining what to do with parts that are already coated but have pinholes in the finish.
What is a pinhole?
A pinhole is a tiny, usually circular, void in a powder coated finish that is visible on the surface of a coated part. During curing, the applied powder melts into a liquid, then spreads into a film, and begins to gel before it is cooled and hardens. A pinhole is caused by a disruption in the gel state of the powder that prevents the powder from achieving a uniform finish. It can be caused by air, water, or oil coming through the powder during the gel state. Air or moisture coming through during the powder’s liquid state doesn’t usually cause a problem because the coating material flows back together and reforms before it reaches a gel state. During any phase, oil can leave a surface tension-inhibiting residue that prevents the hole from closing, causing an issue similar to “fisheye” in liquid painting.
How can pinholes be eliminated?
Diligent Metal Preparation
The metal needs to be clean. It may need blasting, washing, chemical pretreatment, or all of these processes. Make sure dirt and debris have been completely removed. It is critical that no oil or grease is left on the surface. If the metal is really dirty or scaly, like angle iron or castings with mold release on them, preheating the metal to get the oils and silicone out of the pores may be necessary prior to cleaning or blasting. Isopropyl/denatured alcohol can also clean off contaminants right before you spray as a back-up cleaning solution.
Preheating The Substrate
“Gassing out” is a technique used to bring the part to curing temperature before a coating is applied so that anything buried in the metal will escape while there is no powder on the surface that can be disturbed. This is especially important for castings and items that are being repainted. You can apply powder to the hot part or let it cool before applying a coating. If applying while hot, remember that the coverage characteristics will be different than when spraying a room temperature part and the coating will build up more quickly.
Applying The Correct Film Thickness
Make sure you are applying enough powder to achieve at least 2-3 mils of dry film thickness. Use a gauge instead of guessing. A light coating of powder has a tough time spreading uniformly and may go into a gel state faster because there isn’t an adequate amount of powder across the surface of the part. Too much powder can also cause problems. Excessive powder can lead to drips, sags, or a micro-bubble popped surface, so try to stay under 7 mils in a single coat.
Use Clean & Dry Compressed Air
Make sure the compressed air connected to the powder gun is clean, cool, and dry. Compressed air comes in contact with the powder during the powder application process, so if the air is contaminated, the powder will be contaminated. If you don’t have them, install one or more air filters in your system and don’t just run air straight from the compressor to the gun. In addition to causing finish problems, moisture in your compressed air will shorten the service life of some of the components in your powder guns. At a minimum, install drainable filters that can remove some of the condensation that occurs in your air supply piping during humid conditions. The filter system you use should also remove any oil that travels through the air lines. As we mentioned previously, even tiny droplets of oil can cause big finish problems if they get trapped between the surface of the part and the applied powder.
Even if it is oil-free, using hot, moist compressed air can cause finish problems other than just pinholes. An excellent solution is to use a refrigerated air dryer, which dries and cools the air being sent to your powder guns. Here’s a link to an informative article from the Compressed Air & Gas Institute that covers the importance of removing moisture from compressed air.
Keep Booth & Oven Air Clean
Most powder spray booths operate using “negative pressure.” This means that the exhaust system on the booth creates a vacuum inside the booth and fresh air flows into the booth to replace the air that is being sucked out. This is a constant process, so air is continually flowing into the booth and across the surface of the parts being coated.
Dry-off and curing ovens both require the intake of at least a small amount of fresh air to meet various safety codes. Fresh air is pulled through the heater and into the oven by the exhaust fan system. There may also be a combustion fan that draws in fresh air and injects it at the burner. After fresh air enters the oven, it travels over the parts being cured.
Steer Clear of Cross Contamination
With both booths and ovens, shop air is typically drawn into the appliances, often without any filtration. To avoid pinhole issues, don’t spray lubricants or anti-corrosives (like WD-40) near the appliances. Also avoid pressure washing or applying chemical pretreatment in the open shop environment near the booths or ovens. Potential contamination is one of the main reasons to avoid pressure washing parts without using a specialized wash enclosure that has a powered exhaust. The overspray vapor (which may contain detergents or pretreatment chemistry) can migrate through the shop and come in contact with coated but uncured parts. It can also damage expensive machinery over time. Another potential cause of pinholes is the frequent operation of diesel engines near your powder coating appliances. Generators, forklifts, and trucks can all cause problems if their exhaust fumes get drawn into the powder coating appliances and leave an oily residue on parts.
Use The Correct Curing Temperature
While not a typical cause of pinholes, curing powder at a higher temperature than recommended can sometimes create problems. When you raise curing temps in order to shorten process times, you can cause the powder to reach a gel state much more quickly. The faster the powder gels, the less time you have for it to flow and fill in surface imperfections. Usually, this only a problem with thin gauge parts that heat up very quickly. It may be better to lower the temperature of the oven and extend the dwell time instead of risking your finish trying to go fast. You can test a few parts or use scrap material to see how the finish improves with different temperature settings and dwell times.
Try A Different Powder
The powder you are using may not be your best choice if you are having trouble with pinholes. The powder may be too sensitive for the substrate you are trying to coat. By powder sensitivity, we mean the characteristics of the powder that allow it to flow for a very smooth finish. These surface tension additives can be more easily disturbed in smooth mirror-like powders. High gloss black usually has the most issues. Almost all powder companies have a stock gloss black powder and some of these are more forgiving than others. When you encounter a finish problem, having the ability to try similar powders from multiple suppliers can save you time, frustration, and money.
How can I fix pinholes that have already occurred?
Re-Coat The Part
This is the number one fix in terms of saving time and money, but it doesn’t always work. We usually recommend one recoat after heating the part to about 200° F and spraying it while hot. This typically finishes any gassing out that may still be needed and provides a little extra powder to help flow into the holes and hopefully level into a nice finish.
Re-coating doesn’t always work well and it can cause other issues like “orange peel,” but it’s worth a try because the next choice is…
Strip Or Re-Blast The Part
Take it back down to bare metal and start over. No fun, but it always works. Remember that after cleaning the stripped or blasted part, you need to re-bake it and gas it out just in case any moisture or cleaning chemicals went back into the metal pores.
Make sure you only change one thing at a time whenever you are trying to resolve a finishing issue. If you change two or more things before testing on a spare part or piece of scrap, you won’t know exactly what solved your problem.
Need Help? We’re Here for You.
In addition to manufacturing professional powder coating equipment, Reliant also provides a wide range of services to help your existing finishing operation run smoothly–from on-site troubleshooting to training, equipment refurbishment, and more. Call us today at (877) 418-5550.