Weekly Questions and Answers

By George Trigg, GRT Engineering
and Nick Liberto, Powder Coating Consultants

Welcome to Powder Coating magazine's Weekly Questions & Answers column. Questions for this column are submitted by powder coaters just like you who are seeking ways to improve efficiencies and solve every day problems on their powder coating lines.

7/6/2015 - Q: We have a new system that we’re running 8 hours a day. We apply the powder with 10 automatic spray guns. When we start to touch up spots that the automatic guns miss, we see the powder taking on some kind of charge and leaving round spots. It acts as if it has static and wants to drop off the parts. The system is grounded good. Could it be humidity in the air? We have a room that is 40 feet by 70 feet and is kept at 72°F. This is where our booths are at and the powder is stored. We also are running all of our parts in a blaster wash station, a dry-off oven at 300°F, then a cure oven at 400°F for 35 minutes. D.S., Assumption, Ill.

A: What you’re seeing is called “kilovolt (KV) rejection.” It occurs when you hold the gun too close to the part or when you spray too much powder in a given area. I would expect the film build in that area to be quite heavy. A case can be made for doing the touch-up first, then spraying with the automatic guns. You need to run some parts with the automatics only to see where the light spray areas are located. Spray a part with no reinforcement, take a film thickness measurement, and observe the light areas. This will give you a hint as to where the manual application needs to be, and how much. Maybe with some judicial testing you could re-align the automatics and do without the manuals. —G.T.

6/29/2015 - Q: I’ve been trying to perfect my gloss-black parts by using less powder, but my parts still come out with orange peel. I've mentioned to my boss that maybe the curing time is too long and the temperature is too hot. Could you please help? D.A., Wilmington, Dela.

A: In your case, the thinner the film, the more prominent the orange peel. You’ll never be rid of orange peel in powder coating, it's just a matter of how much orange peel you’re willing to put up with on the parts you’re finishing. Your powder coatings supplier can help you diminish the orange-peel effect by supplying you with a fine-grind powder. You’ll pay more for it. You also may have fluidizing and atomizing problems, which can mean a lot of reclaim. As far as the time-and-temperature formula, you should be curing to the powder manufacturer’s specifications, so you really don't have any latitude that way. —G.T.

6/22/2015 - Q: I’m just now getting ready to buy some powder coating equipment for our business but haven't yet found a forum that answers a nagging question I have. Can clear powder coating be done on aluminum on which I have already painted some custom designs using urethane automotive paints? I want to replace the urethane clear coat I’m currently putting on the artwork with a powder coat clear to make the pieces more durable. The paint is stuck very well to the aluminum, but I just wonder if the powder coating process will damage the detailed paint work that is already in place or if the clear will even stick to the painted areas. I’ve also read that there are various clears, such as 85 percent, 95 percent, and so on, but I want it as clear as possible. P.C., Johnstown, Colo.

A: There is no way of knowing beforehand if you’ll have problems coating your artwork with a clear powder coating. These powder coatings are often used over liquid basecoats. However, the liquid coating may cause solvent pops in the powder coatings, depending upon the solvent content and the drying time. The liquid paint may discolor during the powder cure process. My best advice is test a few powder coatings over some scrap pieces to verify results before you destroy a unique piece of artwork. —N.L.

6/15/2015 - Q: Who has the closest looking powder to a true chrome finish? I manufacture automotive aftermarket accessories and would like a good alternative to chrome-plated products that would be more environmentally sensitive. D.L., Commerce, Calif.

A: Most powder coatings suppliers offer the material you’re looking for. Some of this stuff looks really good, actually defying you to distinguish it from real chrome. Don't accept just anything they send you. And don't be satisfied with their panels. When you see some color panels that look good, ask the powder supplier to give you a small sample to spray it out yourself. Some of this material looks great but is a production pain to apply. If you need a list of suppliers, go to www.pcoating.com and click on Online Buyer’s Guide. —G.T.

6/8/2015 - Q: I have metal boxes powder coated various colors. Can I recoat them in a darker color? Is there something I should do first for the powder to adhere? L.D., Ludlow, Mass.

A: You can powder coat them any color you wish. Be aware of some of the following issues. If the original coat is a high-gloss, you’ll need to scuff-sand the entire part. If the original paint is a low-gloss, the only thing you’ll have to do is make sure the surface is clean and free of dirt, oil, and stuff like that. Of a bit more subtle nature is the fact that the second coat will have a fairly pronounced orange peel. It's just the nature of powder coating. Also note that the second coat can give you a total film thickness that could interfere with mating surfaces or other pieces that have to fit together with a tight tolerance. The second coat will be of slightly lighter film build because of the insulating factor of the first coat. So, if you're powder coating a different color, be sure to inspect all appearance areas for good hiding of the first coat. —G.T.

6/1/2015 - Q: We powder coat office furniture (filing cabinets) and always use a hybrid powder coating for interior use. Sometimes after some of the parts have cured, there will be a flow from the powder that bridges the gap where the two pieces of steel are welded together. I notice it more if the film build is getting heavy, up around 3.0 mils to 3.5 mils. Sometimes, it's on a recessed area where two separate pieces meet, and sometimes, it's a flat surface where two pieces are welded together on a small offset. It usually happens when we're coating white or a light silver where we require a heavier film build.

Second, how opaque should a light-colored hybrid powder be? I know it depends on how light the color is, but I'm talking very light, about the color of printer paper. P.L., Pembroke, Ont.

A: Thanks for reading the column. I hope we do some good and help reduce the frustration that comes from operating a finishing business. If I have the picture of your problem somewhat correct, then maybe I can offer the following. I see the likelihood of two issues. If you have any cracks or irregular surface joints, the powder won't bridge them. Instead, it’ll leave a line or a void. If you get lucky, heavy film might camouflage it. Like any paint though, powder isn't designed to cover even the smallest of gaps. In fact, the powder will tend to have edge pull and will pull away from the edge of a crack or irregular surface. You can have the powder modified for less edge pull, but it won't fix it so that it's never there. The other issue is welding. Welding leaves a residual. The severity of it depends on the kind of welding, which has to be sanded or etched or ground away for the paint to get any adhesion. So, if you're seeing pull-away from the welded areas, I suspect that the immediate surface of the weldment isn't properly cleaned. By the way, laser cutting will do the same thing.

As for your second question, opacity is the capability of a coating to cover or hide, especially as in the covering of a substrate or previous coating. OK, now if I interpret your question correctly, you're wondering how well a very light color should hide the surface to which it's applied. It should hide the substrate no matter what the color/pigmentation is, providing the powder is applied at the manufacturer's recommended film thickness and providing you have at the very least a continuous film. A continuous film won’t hide the substrate, but it will be well below whatever film thickness you need and is recommended. At 2.0 mils, for example, you shouldn't see any substrate. If the powder is designed to cover at 1.5 mils, you shouldn't see any substrate, whether the tint/color is white, black, or in between. It shouldn't make any difference if it's a hybrid or a straight material. —G.T.

5/26/2015 - Q: What methods are used to control the finish color of metallic powder? I'm aware of the 3:1 ratio for consistency of virgin powders to reclaim. I've also read much about metallic powder consistency of color from one batch to next not being stable. What in-process methods are employed to maintain a consistent product with metallic powder? From a quality control point of view, what are typical standards for color match and gloss for metallic powder finishes? C.M., Worcester, Mass.

A: The pigmentation of a given metallic color is tightly controlled, as it would be in any other finish color. The problem with metallics is the "metal" flake that gives this finish the metallic look. It used to be that the metal flake in metallic paint was an aluminum or some type of nonmetallic material serving as metal flake. Electrostatics, which are the most efficient means of spray painting, caused the metal flake to stand up on end so to speak, especially in powder coatings. This was hardly an issue in wet coatings, but it was a huge problem with powder coatings. As a result, attempts have been made to substitute a nonmetallic flake for the same purpose but one that wouldn’t be an electrostatic antenna. Of course, this rearrangement of the metal flake gives an entirely different appearance to the finish, mostly unwanted. Mica was one of the substitutes, for example.

There are still issues with metal flake, but the powder suppliers have done a good job of reducing the problems. However, reclaim resurrects appearance problems because the reclaimed flake now contains a bit of the electrostatic charge, as does all reclaim, and it won’t react on the part being coated, quite the same as virgin material. This causes a color shift or the perception of a color shift. Not using reclaim isn’t an option if you’re spraying large quantities of powder. You might reduce the color shift by reducing the voltage slightly when you’re spraying a mix of reclaim and virgin. Beyond that, you’ll have to talk with your powder supplier to see what can be done to help.

By the way, most powder coating systems have a reclaim-to-virgin mix ratio of 60 percent virgin to 40 percent reclaim. If you can or are getting 70 percent to 30 percent, you’re doing great. I would suggest that you verify that the ratio is 3 to 1. I would be impressed if it is. And this difference may be what is causing your color shift problems.

Keep in mind that the powder coatings supplier is taking out anything below 5.0 microns as part of the classifying process. Powder particle size is based on a bell curve, with the tip ends of the bell on both sides removed. If your powder is standard grind, the average micron size is probably about 32.0 microns. Most powder suppliers remove particles 5.0 microns or less from the lower end and particles 55.0 microns or more from the higher end. The velocity of the booth air will erode some of the overspray particles as they come in contact with objects along the path to the reclaim hopper. The very large particles will readily take a charge, so very few large particles make it back into the reclaim hopper. If you have a sieving device, you can begin a program of reducing the screen micron size until the screen begins to plug. In a manufacturing atmosphere, you don't want screen that is too fine because of this packing condition, which will overload the sieving device. If it’s electrically powered, it’ll cause a load on the motor, which causes heat, which will eventually cause the powder to solidify. Then you really have a problem. Hope this helps. —G.T.

5/18/2015 - Q: I’m replacing galvanized sheet metal on a cooling tower. The metal will be subjected to water with biocides, algaecides, and metal corrosion inhibitors in it. In addition, at any given time it may be submersed, drying or wet from splashing. Would powder coating be a way to rust and corrosion proof the metal to lengthen its service life? B.O., Richmond, Va.

A: Depending on the quality of the galvanizing, powder coating may not provide a significant improvement to protect the steel from corrosion. Normally, hot dipped galvanized steel is powder-coated to improve the aesthetics of the part more than to significantly improve the corrosion resistance of the design. Galvanizing applies molten zinc to the steel surface. This zinc is designed to be sacrificed to save the steel from corrosion. Over the life of the product, the amount of zinc lessens as it degrades (like a dissolving bar of soap). Applying powder coating to the surface can inhibit this method of protection. Eventually, the powder coating will delaminate from the steel as the galvanized layer dissolves. When this happens, there is often enough galvanizing left on the steel surface to provide continued protection for some time to come. It’s true that the powder coating will protect the galvanizing from immediate corrosion effects. However, the product’s aesthetics will already be compromised as the coating delaminates and exposes the underlying galvanized surface, often in spotty areas. I hope this information helps. —N.L.

5/11/2015 - Q: The powder spray section of my powder coating line contains a cyclone recovery and spray-to-waste system for short runs. Before discharge to atmosphere, both of these filter through a water scrubber system that is supposed to remove all airborne powder before venting. The problem is the water in the scrubber is frothing badly, and the froth is taken through the system to atmosphere. This froth contains the powder color and thus is settling on vehicles in the parking lot. What is causing the frothing, and how can I permanently stop it? I am currently spraying the froth with mineral turps to minimize it. A.G., Smithfield, New South Wales, Australia

A: The chemistry of the powder, much like the chemicals in the pretreatment system, is reacting with the water and turbulence, and causing the frothing. The only way I know how to prevent this is to buy an anti-foaming agent from a chemical house, such as your pretreatment company, and add it to the system. You can get equipment that will make this addition automatically so that you don't have to be concerned about it. Water suppression systems are very unusual in the US, so we don't see much of what you’re experiencing. We would have a problem getting rid of the contaminated water, but we can spray to waste, sweep up the overspray, and then legally dispose of it at a waste landfill. —G.T.

5/4/2015 - Q: I’m trying to get powder to stick to corrugated e-flute. It works fine on the flat, but of course, it drops off vertical surfaces. Any tips on how to get it to hold on verticals before it gets in the oven? L.H., Camarillo, Calif.

A: I had to look up e-flute online first to answer your question. What I found was that e-flute is a type of corrugated cardboard used in box manufacturing. Because the substrate is paper, the electrostatics used to attract and hold the particulate on the surface are useless and the paper is non-conductive and ungrounded. Therefore, you can try wetting the paper to provide some surface tension to hold the particles or spray a conductive material on the paper before applying the powder. The conductive material that is normally used for this process is called “Rans-Prep” and is applied by using typical liquid application equipment. Good Luck. —N.L.

4/27/2015 - Q: What’s the average shelf life of a powder coating? S.S., Gainesville, GA.

A: The industry standard is usually 1 year. This standard is qualified by the conditions under which the powder is stored and the formulation. Low-cure-temperature materials won’t like a year of storage but may be OK. Uncontrolled atmospheric conditions can cause the powder to age quickly, depending on the ambient temperature of the storage area. I’ve seen powder that couldn’t be used when it was 6 months old. On the other hand, I’ve seen and sprayed powder that was almost a year and a half old that looked OK. —G.T.

4/20/2015 - Q: We’re a project management firm that oversaw the installation of a new powder line, and we’re trying to help our customer. What are some of the causes of intermittent pinholes on a finished product? We’ve looked at time and temperature, rate of temperature rise, powder thickness, and de-ionized rinse. The problem doesn’t occur on the older powder line but does occur on the new line. Any suggestions? The parts with pinholes are cast iron. Aluminum and light sheet metal parts on the same hook look beautiful, so we think the pinhole problem may be due to casting porosity or inadequate drying. It could also be due to the heat-sink nature of the casting, which may not be cool enough before the application of the powder. It’s the intermittent nature of the problem that is puzzling. I appreciate any suggestion no matter how simple or obvious. There are no egos here. T.C., Thornbury, Ontario, CD.

A: At first blush, it would seem that you have an out-gassing problem. Any type of casting, because it’s porous, will out-gas. There are ways to minimize the problem, but all of it may never go away. The powder supplier can modify the powder to gel and cure in a way that prevents the air pockets from getting to the surface. Heavy film on cast parts can help reduce some out-gassing. And preheating the parts to drive out the gas or liquid before coating will help. A lot depends on the porosity of the casting. Don’t preheat the parts and then put them through the washer. That defeats the preheat conditioning. The parts should be 100°F or lower before applying the powder; otherwise, you will build unnecessary film thickness, which is a waste of powder and money. In addition, warm or hot parts won’t build an even film, but will build an erratic film. Infrared curing on the casting could help because it cures only the powder and immediate part surface. However, if you just installed a new line, your customer will likely scream if you suggest some additional equipment. Coating castings has always been an issue with powder coatings. Liquid coatings can also suffer from this condition, but liquid flows out differently than powder and will mask the bubble. —G.T.

4/13/2015 - Q: I have a customer who is asking about the performance of powder coating at -40°F. We’re coating coil springs (high carbon steel) that are cleaned before powder coating through an aggressive shot peen necessary for spring properties. We currently are using silver TGIC-based (triglycidyl isocyanurate) polyester with a full-gloss, clear TGIC-based polyester topcoat. Have powder coatings ever been tested at such extreme temperatures? Any educated guess as to what would happen at this temperature? The customer is being rather evasive about the specific end use. I appreciate any light you can shed on this issue. P.M., Pittsburgh, Pa.

A: In one of my former lives, I worked in the appliance industry. We installed one of the first major automated powder systems for refrigerator liners, which is the interior shell of a refrigerator. This liner is designed to include the freezer portion of the unit. Temperatures in a freezer compartment can reach nearly zero, depending on the setting by the homeowner. We had absolutely no problems. We deliberately flexed the metal to see if we lost adhesion, and everything was OK. Now, I hasten to add that these units are sheet metal and not spring steel; they don’t see daily flexing such as a spring might. I tried cryogenic hook cleaning for a system that I installed, also in the appliance industry. This is a process that was being promoted by a large supplier of the gases used in this process. Couldn’t touch the powder on the hanger. Parameters were about 3 mils of powder; time ranged from a few minutes up to just short of an hour. At about an hour, we saw some cracking and crazing of the outer coating, but it wouldn’t peel from the hanger, even with a hammer blow. This process has been used on liquid paint, at fairly low film, and works. I don’t know how well it works over time. think it’s interesting that you need to put a high-gloss clear over a metallized first coat on a spring. Oh well, none of my business. I sure wouldn’t do anything unless I tested it first. One concern would be the inner-coat reaction between the clear and the silver. The other is, of course, can the coating flex along with the spring steel. Having been intimate with the refrigerator tests, I’m inclined to say it will work OK, at least for a time. Constant flexing, day in and day out, may be something else. I queried a technical director for one of the larger powder coatings suppliers who said that unless the clear is formulated for 0-T, which means the capability to withstand a sheet metal bend back on itself, it’ll craze when the spring does a tight compression. I wouldn’t commit to anything until the customer revealed some additional information. —G.T.

4/6/2015 - Q: I coated a fancy decorative metal piece with a polyester clear that bonded with aluminum. I decorated it in the backyard. It got wet during the last rain, which changed the appearance. It looks like the aluminum oxidized. It looks darker. Is there any way I can restore its original color by cleaning with alcohol or another cleaner? I really appreciate you answer. P.R., Midland, Tex.

A: At what temperature did you cure the clear coat? Aluminum forms a self-protective coating that acts as a preservative. If you didn’t clean that off just before painting, then you will not have any adhesion. If that’s so, then moisture likely got between the clear coat and the aluminum. If the coating was cured at its recommended temperature, alcohol won’t remove it, even with poor adhesion. What may happen if the adhesion is poor? If you try to remove the paint with a mild solvent, some of it will come off and some of it won’t. You need a much stronger solvent, like methylene chloride (MC) or methyl ethyl ketone (MEK), but both of these are nasty to work with manually. Take the proper precautions. If you live near one of the larger cities, try to find a company that does stripping and have them do it for you. It’ll cost a little bit, but it’ll be so much easier than hand stripping. —G.T.

Further reading on the problems discussed in this column can be found in our Article Index and Bookstore.

George R. Trigg is president of GRT Engineering, 6314 Hughes Road, Prospect, OH 43342; 740/494-2496. He has been involved in the powder coating industry for more than 38 years. He holds a BSBA degree from Muskingum College, New Concord, Ohio. His email address is molly95@earthlink.net.

Nick Liberto is president of Powder Coating Consultants (www.powdercc.com), a division of Ninan Inc., 1529 Laurel Avenue, Bridgeport, CT 06604; 203/366-7244. He has more than 3 decades of experience in the powder coating industry. A registered professional engineer in Connecticut, he holds a bachelor’s of science degree in mechanical engineering with a minor in physics. His email address is pcc@powdercoat.com.

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