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.

11/24/2014 - Q: For the past 6 years, I’ve been outsourcing the powder coating of the product that we manufacture. Our product is a 6061 aluminum and is used outside. It’s installed from the beaches of Florida to Hawaii. My question is what type of prep should be done to the old powder coating if we decide to repaint on site? Could it be power-washed hard or should the old finish be removed? If the old finish is failing, what is the new paint holding onto? How can the old powder-coat finish be taken off at someone's home? I’ve looked into a high performance water-based epoxy primer and finish coat recommended for aluminum and galvanized metal. Do you have any recommendations? What are your thoughts on the subject? B.E., Collegeville, Pa.

A: Right up front, you must understand that you have to properly prepare the aluminum before powder coating. Sandblasting won’t do it, especially in a salt-laden atmosphere. If the powder coating is flaking off the aluminum, it’s caused by little or no metal prep in that area. The aluminum must be properly cleaned and conversion-coated immediately before a good quality triglycidyl isocyanurate-based (TGIC-based) polyester powder is applied and cured to the recommended cure cycle. That is metal at temperature, not just oven time. If this action requires some awkward handling of the part, so be it. That’s better than the time and cost of rework.

Now, how are you going to do the field repair? You must sand away the blistered area and get back into the paint that has good adhesion. Then, you’ll need a field metal prep kit, which you can get either from an automotive refinish store or from any large pretreatment company. After following the manufacturer’s recommendations, apply a two-component liquid finish. This is also available at your automotive refinish store, or you may find it at a marina that does boat repairs. DO NOT use a zinc-rich primer. The products that you mentioned are good, but if I’m going to have to go to the field to do repairs, I want to do them only once. The two-component materials will keep you from making a future return trip. —G.T.

11/17/2014 - Q: We coat pipe with powder coating. We have to put 8 mils to 12 mils of powder on the pipe. The pipe sizes range from 1.05 inches to 2.875 inches outside diameter. We have good results on the smaller size pipes, but on the larger sizes we’re having trouble with the powder coating blistering after the pipes come out of the curing oven. Do you have any suggestions, or do you need more information? Please let me know. M.K., Shiner, Tex.

A: I don’t have much to go on here to provide an accurate diagnosis. However, since you’re stating that the problem is exhibited after the cure oven, it may be related to your cure cycle time. Because the larger pipes have more mass, they’ll require a longer cure cycle time. The cure cycle time is equal to the metal bring-up time plus the cure dwell time from the powder supplier (cure cycle time = metal bring-up time + cure + dwell time). Perform an oven profile test with the large pipe and your current oven cure cycle time. Verify that the part is at metal temperature for the prescribed time specified by the powder supplier. If not, adjust your time or temperature accordingly. —N.L.

11/10/2014 - Q: We manufacture pump tanks that are installed inside or outside, depending on the installation. We currently powder coat our tanks with a film thickness in the range of 2.0 mils to 2.5 mils. We’ve been questioned on why we go so thick with our coatings, as we’ve been told that 1.0 mil to 1.8 mils should be sufficient. Just wondering if our thicker coating offers any rust-protection advantage, or are we just wasting powder? Our powder coating is applied on cold rolled steel shells that have been prepped with zinc phosphate. W.S., Stratford, Ont.

A: All things considered, you’re giving your customer more than he’s paying for and more than is needed. There will be some added benefit to the heavier film, given adequate cure. It’ll be a bit more difficult for anything to penetrate the surface, but that benefit could be subjective. If you can lower the film thickness a tad and maintain the uniformity of the coating, you would be a better business person. That statement means that you’re giving away some powder on each part that’s coming out of your profits. Depending on how many parts are involved, you could be talking about a nice piece of change. Your powder formula will indicate the best coverage for the material you’re using. I would be a little concerned about a 1.0 mil or lower limit. That thickness courts trouble in case something goes wrong, which is inevitable. I would shoot for a 1.4-mils minimum to a 1.8-mils maximum. This will do nicely, especially over a zinc phosphate, and the orange peel should be minimal. —G.T.

11/3/2014 - Q: We have issues with nylon powder coating drawing up from the corners of parts. We preheat the parts and apply the powder so that it adheres to the part surfaces immediately. Occasionally, however, the coating will shrink and draw back when the parts are curing in the oven. T.C., Glasgow, Ky.

A: Do you have a primer on these parts before application of the nylon? You didn’t mention it, so I’m not clear on that part of the process, but that could be the problem. And too much heat may cause some pull-away from the edges and corners. So, maybe you’re trying to use the preheat in place of the primer. Any irregularity in parts, such as thickness, and any change in conditions could cause a sporadic cure problem. Maybe the preheat temperatures are inconsistent. Maybe the oven temperature or air circulation isn’t the same every time. How old is the nylon powder? I am unaware of any real issues with shelf life providing the powder is used in a reasonable time and stored in conditions dictated by the supplier. —G.T.

10/27/2014 - Q: We’re considering options for staging multiple stand-alone carts that will hold multiple pieces that are being powder-coated. After the wash process is complete, we want to load these stand-alone rolling carts with work. Because these carts aren’t attached to a conveyor and grounding is necessary for the powder to stay attracted to the work, is there a stand-alone device that we can attach to each cart to create grounding? Another option would be to attach a rolling cable from the conveyor to the cart; however, we need more flexibility with staging of work. Your input will be greatly appreciated. E.N., Midland, Tex.

A: The simplest and cheapest thing you can do is attach a spring-loaded alligator clip on the other end of a ground cord to the powder system. When you wheel the cart to the application area, just clip the ground wire to the cart frame. The ground wire must make a complete circuit with the spray gun to be effective. I don’t know of a device, per se, that can do that any better. Having said that, if the floor of the spray area is metal, and if the carts have metal wheels, you might get a ground that way. That’s a lot of ifs. The worst part is there are a lot of contact points here that can cause loss or reduction of ground, so this method isn’t the most reliable method to use. You must surely know that spraying the parts while they’re on the cart is a sure way to get a lot of powder on the cart. Eventually, the cart should be cleaned. How will you do that? Or maybe the cart is small? —G.T.

10/20/2014 - Q: I have two sets of wrought iron gates that are newly fabricated and galvanized — two 3-foot-by-8-foot panels and two 3-foot-by-4-foot panels. The subcontractor brought it to a powder coater to get it painted, but then informed us that the paint has all these bubbles and holes in it after coming out of the oven. The general contractor brought it to another painter who said the same thing would happen. What are they doing incorrectly or what do they not know in order to powder coat these galvanized wrought iron gates properly? I understand that there can be issues, but that they can be mitigated with the correct surface preparation methods. Any advice or recommendations? How about using an epoxy sealer before powder coating or trying special degassing powders? Our general contractor has all but given up on getting these done. Help would be much appreciated. J.D., Lakeland, Fla.

A: Powder coating over hot dipped galvanizing is a sketchy proposition and results can vary widely. Hot dipped galvanize is an unstable substrate for powder coating as outgassing normally occurs during the cure process. Some people have success when powder coating within 24 hours after galvanizing after performing a quick brush blast to remove surface contaminants and preheating the parts in the cure oven above the cure temperature for twice the cure time before coating to drive out internal gasses. However, this methodology does not always produce predictable results and you may still have bubbles and pinholes in the coating. As the galvanizing ages, these problems can get worse, since the galvanizing can absorb moisture over time and the brush blast may not remove all the surface soils. However, galvanizing that has aged up to 2 years can become more stable and easier to coat without pinholes, but still requires aggressive cleaning to remove the zinc oxide. All these stages and cleaning methods are outlined in ASTM D 6386, a specification for preparing hot dipped galvanized parts for painting.

Please realize that powder coating over hot dipped galvanized steel does not provide any appreciable corrosion resistance, it just makes the product look more appealing. Galvanizing protects ferrous substrates by sacrificing itself to protect the substrate. Since the hot dipped galvanizing is often applied at 12 mils or thicker, this can result in 30 years of protection, depending upon what conditions the product sees. This sacrificial reaction with the elements will occur even under a powder coating. However, when the sacrificed zinc oxide material is formed and “sloughs off” the part, it will carry the coating with it and expose a new layer of galvanizing. This situation will require constant maintenance and recoating (or at least touch-up) over the life of the product, whereas galvanizing alone will provide similar corrosion protection without any maintenance over the product life, as only the patina of the zinc will change. This has been the subject of many a lawsuit, as customers expect maintenance-free product life from galvanized products but want them to look more aesthetically pleasing by applying a coating over the galvanized surface, resulting in “premature” coating failure and costly repainting. —N.L.

10/13/2014 - Q: What is crazing and what causes it? We clean and then send printed (screened ink) aluminum sheets, 0.018 inch to 0.030 inch thick, to a powder coater. This is a polyester powder coat applied at 1.5 mils to 3.0 mils thick. When the sheets are returned, we machine them and run them on a belt through an oven at 280°F to remove "blushing.” Unfortunately, the parts are crazing. To stop the crazing, we have to run the sheets before forming through the oven first. Sometimes, we have to run them through a hotter oven at 400°F. Any idea what causes the crazing? My understanding is that crazing is small hairline fractures in the powder coat. If we ship parts that show crazing to our customers, and they are applied to vehicles, are they susceptible to corrosion? M.H., Munich, Germany

A: Maybe. If you’re lucky, it will not go to bare metal, but I would not bet on it unless I looked closely at the cracks under a glass. It can depend on what is causing the crazing. It can be caused by too much heat. It can be caused by surface tension between two coatings that aren’t compatible, and on occasion between the substrate and the coating. The 280°F oven temperature is almost a 100 percent overbake of the powder. I wouldn’t think that would cause the material to craze, but you never know.

You should check for compatibility of the two coatings with one another. One way you can do that is to use cold rolled steel test panels. Spray on the base coat, cure it out, apply the screening or the second coat, cure that, and then take a look. Use a glass to be sure there aren’t miniscule cracks. I could also make a case for the thin sheet aluminum becoming "active" during the bake cycles and causing the coating to move when it doesn’t want to. The aluminum won’t corrode, but it will develop a patina coating where it’s exposed to air. More important, if the cracks are to the substrate, you now have an entry for moisture to get under the coating and cause it to blister or lift. The aluminum will be safe, but the part will look bad and cause complaints. —G.T.

10/6/2014 - Q: Our powder coating operation is currently facing a problem with impact test failure. Does zinc phosphate pretreatment play any role in this type of failure? If so, how do we get our coating weight in the correct range to pass the impact test? A.M., Navi, Mumbai

A: Impact fusion is a mechanical property that is formulated in the powder coating. Once the formula is made you can only degrade this maximum level of impact by process problems. Therefore, the first thing you need to do is to verify that your powder coating formula is capable of achieving the target impact resistance (direct and indirect) you want to attain.

After you have verified that the formula is capable, then you can look at process issues. The most common process issues that affect impact and other mechanical properties are under/over cure, high coating thickness, and poor pretreatment/cleaning of the substrate. All of these process related issues must be performed within tolerance for the coating to perform under mechanical stress.

You specifically asked about your pretreatment (zinc phosphate) as being the root cause of your impact failures. The only way zinc phosphate can affect coating impact performance is if it has been applied too thick (too high a coating weight). If you stay under 140 mg/square foot you should have no problems with impact resistance caused by the zinc phosphate. For reference, iron phosphate can be applied up to 70 mg/square foot before it can be problematic. —N.L.

9/29/2014 - Q: I need to ask a question that I’m sure everyone who applies powder coating is struggling with. Do you know how to eliminate the extreme flashing around threaded holes when plugged with a silicone plug? The buildup that we’re seeing is with heavy-mil powder coating. If anyone knows of a special plug system, or another way of masking these holes, I’d greatly appreciate it. J.M., Churchville, N.Y.

A: Other than experimenting with masking from a dozen suppliers, the only suggestion I’d have is to use a very small vacuum line to remove the powder that is at the bridge between the current thread mask and the body of the part. This can be labor intensive unless the situation is consistent and you can automate. There is some commonality in what you’re doing, yet each company has its own part design, and therefore, a lot of time is spent in experimentation. —G.T.

9/22/2014 - Q: We’re using forced air (fans and portable evaporative coolers) to cool our product after it leaves the oven so that it can be packaged, a process done by hand. We’re having a problem getting the product cool enough to handle when the ambient temperature rises in the summer months. (The plant isn’t environmentally controlled.) I was wondering if using chilled air would work better. Will it affect the product by interfering with the curing cycle? What are your thoughts on this? Thanks. I’m learning a lot by reading your column. R.B., Harvey, Ill.

A: The powder coating should have reached its cure-time-and-temp cycle before it leaves the oven. It won’t post-cure as a solvent-based material will. It’s common to use chilled air for cooldown after the oven. This won’t be the cheapest thing to do because you’ll need some type of chiller. Make sure there aren’t any chilled air currents getting to the oven that will affect temps inside. This shouldn’t be an issue but just a note about things to be aware of! Should I assume that you have used fans to aid in the cooling? Forced-air moving across the parts will do wonders in lowering the temp of the parts. —G.T.

9/15/2014 - Q: I’ve been trying to powder coat a set of 350 Chevy heads. When I shoot the heads with black powder and then cook them, the powder isn’t flowing out. It comes out as if it were never cooked. How can I get the powder on the parts to flow out? Are the heads soaking up the powder? Please help me with this. I’m baking at 500°F. C.Y., Maryville, Tenn.

A: The 350 C.I.D. Chevy motor is very reliable. I had one in several vehicles over the years. However, I prefer the horsepower or the higher revving 327 C.I.D.

The description of the problem you’re having tells me that you’re not heating the cylinder heads to a point where the powder coating even melts, let alone cures. If the powder on the part looks the same when you take it from the oven as when you put it in the oven (still a powder and not a coating), then it’s time to evaluate what you’re using to heat your part.

This problem has to do with the weight (mass) of the cylinder heads and the energy capacity of your heat source. I’ve lifted my share of Chevy heads over the years, and from my recollection, they weigh about 100 pounds. Considering that the cylinder head is made from steel and has a specific heat of 0.125 BTU per pound, it will take 4,375 BTUs to heat one cylinder head to 350°F (the average cure temperature for powder coating). If you put this cylinder head into an oven whose heat source has an energy capacity of 1,000 BTUs per hour, it would take 4.375 hours to get the cylinder head up to temperature and an additional 25 minutes to fully cure the powder coating. That’s almost 5 hours of baking time! However, if you use an oven with a heat source that has an energy capacity of 10,000 BTUs per hour, it would take less than an hour. This relation between the energy capacity of heat sources and cure time can easily be related to horsepower and speed: The more you have, the faster you’ll go.

I frequently tell my clients that the size of their oven burners determines how fast their products will achieve the desired powder cure temperature. I often tell them that you can cure a 10,000-pound part with a cigarette lighter. It will just take a couple of hundred years. The same goes for you. If you’re trying to cure your cylinder heads in a toaster oven, then I hope you’re very patient because it will take quite a while to melt, flow, and cure the powder. Remember the old racer’s adage: “Go big or go home.” Of course, they were talking about engines (horsepower), but you get the point. —N.L.

9/8/2014 - Q: We don’t have the capability to powder coat enclosures at our plant, but we do buy powder-coated enclosures. We try not to scratch them, but scratches happen. What is the best way to fix these scratches? They can range from a small scrape with a piece of metal to larger dings with a forklift. S.F., South Burlington, Vt.

A: You can use a color-matched liquid paint for almost all of the touch-up. This is an accepted industry standard. Now here are some cautions. Try not to sand down to bare metal. If you can do that, then you can maintain the integrity of the pretreatment. If you have to go to bare metal, then you should use some type of bottled chemical treatment on the metal before applying the coating. You can find this material in an automotive paint supply house. You can recoat the entire part if orange peel isn’t a problem or if close tolerances don’t present difficult fits with something. If the original powder is high-gloss, it will have to be scuffed or you’ll have poor adhesion. If it’s a low- or medium-gloss, just make sure the part is dust- and oil-free, and you should be OK. —G.T.

9/2/2014 - Q: I’m having fisheyes or pitting after powder coating. What is the cause of this? Am I not cleaning the parts properly? J.B., Seattle, Wash.

A: The most prevalent cause of fisheyes is contamination. Contamination can come in many forms. It can be on the surface of the part due to poor cleaning. It can be oil in the compressed air used to pump or fluidize the powder. It can be an airborne contaminant, such as silicone or some solvent, in the plant. Find the source of the contamination and the fisheyes will go away.

Surface contamination is the easiest one to fix, as all you have to do is clean the part better. Oil in the compressed-air supply can be very difficult to clean, as it will be in all the devices, that is, regulators, gauges, hoses, fluidized plates, gun parts, pump parts, and so on. Cleaning or flushing oil from compressed-air lines is very difficult. Work with your equipment supplier, who may have some ideas on how to do it and what to use. You may have to replace everything throughout the compressed-air system, which is a very difficult and costly chore.

Airborne contaminants are truly the worst to identify. So many things can be the source, from lubricants used elsewhere in the plant to hand creams used by workers on the line. Some people have spent months of time and boatloads of money trying to rid a system of an unknown airborne contaminant. Just hope that you don’t have this problem. —N.L.

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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