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.

7/21/2014 - Q: We make relatively thin gauge open-top aluminum tanks that hold hot salt or chlorinated water. We specified a fusion bond epoxy lining because of our experience with this product on steel piping. Our power coating contractor is having problems getting satisfactory results. Is there a product or process that will give the corrosion resistance of the epoxy that can be applied to surfaces that cool rapidly? The surface needs to be cosmetically appealing and resistant to submersion in hot water. M.S., Surrey, B.C.

A: By definition, Fusion Bonded Epoxy (FBE) describes a method by which a functional epoxy powder coating is applied to an object that is preheated to more than 600°F. The applied coating will melt and flow on contact with the preheated part and cure in approximately 30 to 40 seconds. The applied coating is normally applied thicker than 12 mils (often 20+ mils) using this process. The epoxy is formulated with a fast curing catalyst and is ground to a larger than normal particle size to allow for a quick build of the intended coating thickness.

The coating can be applied using guns or fluidized bed methods, depending upon the application. This coating process is normally used to coat concrete reinforcement bar, pipe, bus bar, and other functional products.

Although this is the definition of the FBE coating process, this does not mean that the methodology cannot be modified to accommodate process issues, like quickly cooling substrates. The thermal characteristics of aluminum are very different than steel. Aluminum conducts heat about twice as fast as steel and, therefore, cools twice as fast. We have many clients who manufacture products with this substrate or other products that cool faster than the FBE applied material can fully cure. Considering that fully curing the epoxy is imperative to achieve full functionality (including corrosion resistance), you must ensure that the epoxy is fully cured on the product. This is why most people will employ a post-application cure step in the FBE process to reheat the coated part up to 500°F or hotter to ensure that the coating is fully cured. How much time the product must spend inside this post-cure oven is dependent upon how much cure is attained at the time of coating application. Performing an oven profile on a test part through preheat, application, and post-heat steps will ensure you develop an appropriate recipe to follow. —N.L.

7/14/2014 - Q: My company’s metal panel surfaces end up with water marks on them after our coating process is complete. For pretreatment, our process includes pre-degreasing, degreasing, hot water rinse, iron phosphate, cool water rinse 1, cool water rinse 2, and DI water. Our cool water rinse 1 pH level is 5, cool water rinse 2 pH level is 6, and DI water pH level is 7. Please help. Z.X., Selangor, Malaysia

A: Water spots are caused by minerals in the rinse water or when the cleaner stage did not provide a water-break-free surface. DI and RO water generation systems generate mineral-free water that should eliminate spotting issues. However, pH only measures the acidity or alkalinity of the water (with neutral at 6) and does not measure mineral content. Therefore, check the TDS (total dissolved solids) or conductivity of your water to determine mineral content. This measurement, along with a complete laboratory analysis of mineral content in your water will tell you if your DI or RO system is working properly. If the spotting is caused by an ineffective cleaning stage that does not provide a water-break-free surface, then fix that with better chemistry, heat, or mechanical force. A water-break-free surface is by definition spot-free (no beading of water), and therefore will not have any spotting issues. It also means that surface is organic soil free, an important condition to ensure proper coating adhesion. —N.L.

7/7/2014 - Q: I’m looking at a rusting issue with our powder-coated products. In the fabrication process, we spot-weld several small items, such as hinges, to the surfaces of our product while in the raw state. Once this equipment is placed in service and exposed to water in the form of rain or otherwise, rust stains will begin weeping out from between the hinges and the main product. We do not wish to use primer paint before spot-welding because of extensive handling and cost. Is there a product that can be wiped or brushed onto the parts to achieve a higher degree of rust protection than we’re getting now and won’t impede spot-welding performance? We use a wash-rinse system to place an iron phosphate coating onto our carbon steel parts before powder coating. A.R., Fayetteville, N.C.

A: This is a very common problem with two metal pieces joined together via welding. At an automotive refinish store, you can get a zinc-rich material that is designed for application on bare metal to act as another barrier to rust. This material will accept paint. You can swipe this on with a rag or brush or cotton swab. Do this after the spot-weld operation. Give it time to dry and then paint. Also, give the parts a few seconds to cool after the spot-welding. Maybe you can work it in so that the guy spot-welding can also perform this duty. This seems to be a very successful application. At least I have never been told otherwise, not to my face anyway. —G.T.

6/30/2014 - Q: I’m using textured powder. I need a uniform appearance in texture on the entire part and a film thickness of 4.0 mils to 8.0 mils. When the part comes out of the powder coating line, some areas look different in texture. (I can see different texture on the part--heavy versus normal.) But when I measure these two areas with different textures, they have the same film thickness. What could be the reason or root cause of this issue? A.R., Lithonia, Ga.

A: You have a film variation of 4.0 mils, starting with a base film build of 4.0 mils, and you wonder why it looks different? You powder guys can correct me, but a textured powder at 8.0 mils is way beyond what it should be. No wonder it looks different. What is the design film build of the powder? Before I get too mouthy, I should ask that. I think you need to do a better job of controlling the film (see your supplier’s film recommendations). Know what the film thickness readings are. Getting a good, true film reading on texture isn’t easy because of the texture. You can read a "valley" at one reading, and then read a "peak" the next time and get confused. Without spending a lot of money on a fancy film gauge, you can take a reading over many locations and average them out to see what the average is. I still think a lot of the problem is the huge difference in film thickness. —G.T.

6/23/2014 - Q: The substrate is 304L stainless steel, B finish sheet. The weldment is a squirrel cage style fan unit with 24 inch wide by 72 inch curved blades. We clean the weldment in a three-stage wash consisting of iron phosphate, water rinse, and rinse seal. Coating is a light green polyester with TGIC (triglycidyl isocyanurate). We have to rotate this fan unit when applying powder to reach all of the areas. This causes us some problems. Also, the visual appearance at the time of application of the powder appears to be covering well (3.0 mils to 6.0 mils uncured). After curing, we have areas that appear light but have a cured mil of 1.5. Any application suggestions? L.H., Carrington, N.Dak.

A: Have you checked the powder supplier to see where the hiding power film thickness is? In other words, if the powder is designed to cover nicely at 2.0 mils to 2.5 mils, something less than 2.0 mils will look blue. As I visualize that part, it seems to me that there are a lot of edges that can rob powder from where you are aiming. All those edges act as antennas and will draw away some of the coating. It goes without saying, but I will say it anyway, the very best film check is on the cured film. That is what you have to go by no matter what it appears to be in the uncured state. If it’s Tuesday and you got a little bit sideways on Monday night, the uncured film appearance might not be what it is! Got that? I don’t envy you trying to coat a squirrel cage fan. It would be time-consuming and full of Faradays. —G.T.

6/16/2014 - Q: We’re looking for a material that could be machined and used as part of our powder racks for masking areas of product where no powder is allowed. Masking parts is too costly. We would like this protection to be part of the racks. We have the capability to machine and build special racks. We have a standard powder process with a five-stage washer. Part temperature is up to 475°F in the cure oven. B.J., Jasper, Ind.

A: Whatever you use will get powder on it and will be baked when the rack hits the oven. I don’t care what you use. Having said that, here are several things to consider. With a nonferrous metal affixed to the rack at the appropriate place, you will be able to get several round trips before cleaning is necessary. If you can, design a "mask" that can be easily replaced and have plenty of them so that you can clean them by the bucketful. The problem is powder will collect on the mask, regardless of the electrostatics. After all, it’s dust, and you know where that goes. Take a look at either vacuum removal or pneumatic removal. A 1/8-inch rubber/vinyl tube can remove a pretty detailed line of powder when properly situated. I don’t know of anything that you can go out and buy that will be the perfect answer. Try one or all of the methods I’ve suggested. —G.T.

6/9/2014 - Q: In our continuing efforts to take variation out of our process, the challenge arises to determine the virgin-to-reclaim powder ratio in our feed hopper. We have asked all our business associates, from powder vendor to engineers who are old school and new, for their thoughts. Still no practical way has been offered that can be used in the process to spot-check and record the ratio. It’s understood that it can be done with laboratory equipment, but a straightforward way to take a sample is what is needed. The idea of weighting was tabled, but it has been determined that the weight difference wouldn’t be measurable and is complicated by where the sample is taken from in the hopper, or does it make a difference where the sample is taken from in the hopper? Another thought is to use a less sophisticated piece of equipment that might serve to just indicate a trend. Can you make any other suggestions that may be tested? C.M., Worcester, Mass.

A: Maybe I should ask why you want to do this. No, I don’t know of a simple shop-related method to determine virgin-to-reclaim powder ratio. The typical ratio would be 60 percent virgin powder and 40 percent reclaim. A higher virgin percent would be just great. Less would begin to cause problems. To be able to measure this scientifically would require a device that could measure powder particle size, and even then, you would have to take random samples and arrive at an average. Immediately after you have added virgin powder to the hopper, you’ll get a slug of virgin-sized particles. After a short time, the fluidization process will blend this with whatever is in the hopper. Reclaim particle size will affect the ratio as the transfer pumps deliver reclaim via the cyclone. You could pull a sample of the reclaim from the recovery system, and also have a sample of virgin-particle-sized powder, and then compare the mix with the previous two. But the same old thing applies: You need a method to check particle size. Some devices out there are supposed to be capable of checking particle size on random samples on the spot, but I have no idea if they work. Don’t make life too difficult. A good trained eye can tell when the ratios are getting out of whack. Too much orange peel, difficulty fluidizing, reduced transfer efficiency, and so on are good indicators. It’s a fact that complex parts will be far less efficient to spray than simple parts. If this is the case, you must plan for a lot of reclaim. And a lot of reclaim means poor transfer efficiency, which leads to a hopper overflowing with powder that won’t like to take a charge. There is the potential here for a complete theoretical discussion on the workings of a powder recovery system. Suffice it to say, I don’t have any experience with any device that you can take out to the shop and determine how much reclaim is in any given hopper. If some company makes one, let me know. I will pass it on, and this reader can give us a full report on its success or lack thereof. —G.T.

6/2/2014 - Q: We’re powder coating energy meter boxes with gray on the outside and white on the inside. Can you please suggest the proper way to mask the boxes so that we can do this type of application? The box is about 250 millimeters (mm) long by 300mm wide by 170mm deep. The depth is achieved by a 100-mm deep bottom box and a 70-mm deep cover, which is joined with the help of hinges to the box. A.D., Ghaziabad, Uttar Pradesh, India

A: Ideally, you powder coat the inside of the box first, mask the opening, and then powder coat the outside of the box. A word of caution: With a full cure on the white, and then a rebake when you cure the outside, you probably will get some discoloration of the white. So, cure the inside just enough to set the material, mask the opening, spray the outside, and then fully bake. If you’re currently doing some of this work and the white is not discoloring, then you should be in good shape. I don’t know of a good way to do this in one operation. The masking system or device could be unwieldy. For instance, you could spray the interior, which has a lot of Faraday cages that result in irregular film, then vacuum off the overspray from the exterior, place a mask (like a door over the opening) while you spray the exterior gray. Then you would cure. I’m not real fond of this process, but it would work. You could try a variation of this by spraying the exterior first, cleaning the interior, then placing a mask over the outside while spraying the interior. I assure you all of these methods will require some experimentation. —G.T.

5/27/2014 - Q: This is a two-part question. (1) Is it possible or probable to have powder coating as a home shop business for small items? (2) Can powder coating be successfully used on stainless steel? These questions refer to a one-man garage type business for small (1-inch by 4-inch to 1½-inch by 9-inch thin sheet metal) pre-manufactured items. Thank you very much for your help. A.L., San Antonio, Tex.

A: Yes to both, with some caveats. Define successful. How much do you want to invest in equipment? That will dictate what kind of work you can do. Don't give up your day job! Hobby coating usually includes a manual spray gun, probably bought used, an old cooking stove with a good working oven, some type of media blasting system, and most likely, a solvent cleaning tank. You need to have some type of "shelter" for spraying. Spraying in the garage will create an unholy mess and may paint the neighbor’s car or yours. With some ingenuity, you can make a booth with a blower and some filtration. It will be real easy to have to spend at least $10,000 in equipment. And watch out for city ordinances.

Are there coating performance requirements for the pieces you plan to coat? You need to have a clear understanding, preferably in writing as to what this coating is supposed to do. If the part is just going to look pretty and scratches don't count, then maybe you can do okay.

If these parts are of stainless steel, then is it high gloss or low gloss? High-gloss stainless won’t give a good adhesive surface for the powder coating to adhere to, so you would need to scuff-sand all the coated areas. Your customer should understand how easily glossy stainless will give up its painted surface. Make your customer a sample so that you both understand what it is you’re getting into. By the way, these adhesion issues with stainless aren’t confined only to powder coatings. They apply to liquid coatings as well. —G.T.

5/19/2014 - Q: We’ve had to put on some 2-inch pieces along the top rail of a trailer. When we go to powder coat it, the spray gun seems to blow the powder off of the inside instead of making it stick. Is there a suggestion on how to eliminate this? We’ve tried to move the gun farther away, and we lowered the airflow. The ground seems to be OK while the rest of the trailer is coated. I’m sort of at a loss here. It’s just a very tight space, and there isn’t a way for us to coat it unless we get the gun down inside of that space. A.B., Hugoton, Kans.

A: Sounds like a grounding problem from here. You also may have the conveying air too high. If the ground is good and attracts the powder as it should, some of the powder should stay on the part unless conveying air is too high, the gun is too close to the part, or both. Same answer though, which is the air is too much, and the ground is too poor. If I’m missing something here, send a little better definition. —G.T.

5/12/2014 - Q: When you have defective parts for whatever reason and you must rework your part, what’s the easiest method for removing the powder coating? Please leave out sandblasting as this is not an option for us because of the gauge and type of material we use. B.K., Mt. Vernon, Ohio

A: My interpretation of your last sentence is that this is a light-gauge material. If I’m wrong, 1,000 lashes. If I’m right, burn-off would damage the material. About the only thing left is chemical stripping. This will remove the powder with no warping of the metal. It’s not a cheap method but neither is burn-off. These systems are no longer the hazard they were 30 years ago. Are the defects so extensive that you must strip the entire part? You can do small items with a liquid touch-up paint, provided you don’t sand to bare metal. You can also recoat the entire part with little sanding, depending on the gloss of the powder. The downside to this is that orange peel will stand out; and if there are any machined-fit parts, you could experience tolerance issues. —G.T.

5/5/2014 - Q: We’re encountering a lot of orange peel when running high-gloss black powder coating. Any idea how to reduce orange peel? A.R., Chatsworth, Calif.

A: If you develop a simple, quick procedure, be sure to patent it because you’ll be on to something that the powder companies will beat a path to your door to buy. The standard for automotive finishes is Lexus black. It’s a liquid, not a powder; however, when you read it on an image meter it’s the flattest finish, without discernable orange peel. You know what a Lexus costs? Part of that high price tag is for the paint. It’s difficult to eliminate or reduce orange peel in a black powder coating. Your best bet is to use a fine-micron-sized powder applied exactly to the manufacturer’s specifications. You will not like the price of the powder. Otherwise, be sure your powder supplier understands what it is you want. And make sure the film build is the least it can be to provide coverage and a continuous film. —G.T.

4/28/2014 - Q: We have several problems stripping multiple layers of lead paint from parts. Most of the parts are 20-gauge steel, but we also strip many irregular-shaped pieces of various thicknesses. Some of the parts are rusted and are glass-media blasted by an outside contractor. The parts are shipped to our powder coater for finishing. The powder supplier wants a material thickness of 1.5 mils. They want a surface profile of 1 mil. With a surface profilometer, we measure a surface roughness of 5.6 micrometers (221 microinches) to 16.9 micrometers (665 microinches). Can this information be converted to mils? Do we already have the correct information and some of our parts are too rough? The rougher profile will fray cleaning rags for wiping the parts with solvent. This traps lint on the part surface, and the lint is difficult to remove. Is there a better material or method for stripping and prepping the parts? J.D., Brookville, Pa.

A: I would look for someone in the general area that does liquid stripping. That way you do away with the roughness left by the blast media, and the liquid strippers will get into all of the corners without any ill effects to the surrounding metal surface. The rusted condition is another problem altogether. "Surface" rusting can be removed; but if the metal is eaten away, the part is scrap, unless your customer will pay for a lot of metal repair above and beyond painting. And don’t offer to do it for little or no cost. You could be taken advantage of in this situation, and you will lose. I don’t know what cities you’re close to, but you should be able to find someone who does liquid 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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