1/30/2012
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Q: How do you check for pinholes in a .010-inch conductive polymeric coating that contains carbon fibers? Spark testing does work. G.S., Colchester, Vt.
A: The best way I know of checking for pinholes, or voids, in the coating is by a dielectric tester. Ground the part and use a wire brush that is attached to the power side of the device. You can run the "brush" over the surface of the part. Any voids that go to bare metal will arc. These can be repaired via a kit that you can obtain from your powder supplier.
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1/23/2012
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Q: Can an anodized roll-up door frame be powder-coated? We wish to have only the inside members powder coated black. Is a special pretreatment required? Please advise on the pretreatment process and the recommended powder to use. D.W., Regina, Saskatchewan, CD
A: Yes, you can powder coat over anodizing. The only cleaning problem is making sure oils or other contaminants are cleaned away. It isn't necessary to phosphate, but a final de-ionized mist would help ensure that any water salts are removed because they will telegraph through, especially with a black coating. I assume that you have figured a way to mask the area that doesn't require the black powder.
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1/16/2012
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Q: I have a set of motorbike wheels for powder coating. The spokes were already painted, and the aluminum lips were exposed when I got them. After sandblasting, I powder coated them and pimples appeared all over the lips, so I sanded them down and did them again. The same thing happened, but not as bad. How can I rectify this? B.W., Dublin, Ireland
A: I don't know for sure what your definition is of pimples, but because it’s an aluminum part, my guess would be that they’re out-gassing bubbles. This would be pretty typical of aluminum. So try this: After sandblasting and before applying the powder coating, place the parts in the oven, bring them up to cure temperature, and hold them there for about 10 minutes. Allow them to cool to ambient temperature, wipe them with a dry clean cloth, and powder coat. An alternative is to apply a thick layer of powder at 8 to 10 mils. This will inhibit out-gassing if that’s what we’re talking about here. The only problem with this method is the heavy coating is more apt to chip, and because the heavy coat would be on the edge of the rim, right where a tire iron would be used to mount and dismount the tire, it’s most likely to chip. So, I prefer method #1. If neither method works, then I would need a better description of the pimples, because they may not be out-gasses.
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1/9/2012
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Q: Can you tell me what's the best way to paint galvanized steel hot? Thanks! A.D., Rivière-du-Loup, Québec, CD
A: I have to assume that if you’re painting hot, you’re trying to build film. There’s no guarantee that you won't get out-gassing during the final cure, but heavy film will certainly have an inclination to inhibit the out-gassing. I need to know what you mean by "hot"? Is that cure temperature, dry-off temperature, or what? Generally speaking, coating hot parts is to build film, and a fluidized bed is good for this purpose. I’d need to know what the part design is to know if spray application would work. You’ll lose some control of film build when coating a hot piece, so you have to decide if that matters.
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1/3/2012
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Q: I have an application in which a carbon steel component will be continuously submerged in water at a depth of up to 5 feet. Will a powder coating provide a waterproof surface? B.S., Idaho Falls, Idaho
A: You can powder coat steel for use in a submerged atmosphere. The US Navy does it all the time. You must have a good pretreatment. The parts will need a pristine cleaning. There can be nothing on the surface that would affect adhesion. The best coating is a heavy film of epoxy. Epoxies can tolerate most anything that would be in the water. And heavy film is crucial. The Navy applies an epoxy to the radar domes used on submarines. This application is at about 60 mils. They build the film in layers, and even under these conditions, the domes require recoating about every 7 years. If you can, coat a sample to about 15 mils and place the piece in a tank of some sort. Check it every two or three days to see how it’s doing. Prepared correctly, the parts shouldn't have any problems.
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12/27/2011
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Q: I’m having difficulty applying an orange, dimple-type texture. It appears to me that the powder isn’t flowing together well, creating what looks like pinholes and allowing you to see the substrate. I’ve tried a flat spray tip and a diffuser tip, and adjusted the spray-gun kilovolt’s per the manufacturer’s recommendation. Today, I noticed that this powder we’re receiving is over 2 years old. Any suggestions would help; I’m getting very frustrated. I’ve checked out the washer, and other powders spray without problems. C.S., Pleasant Hill, Mo.
A: Your powder supplier should have volunteered that the powder is old. Two years is way beyond normal. Any powder, even the most elementary, has a shelf life of about 1 year and that depends upon the conditions in which it has been stored. Controlled atmosphere is critical, the older the powder, but even so, anything older than a year is subject to all kinds of possible defects. I would insist that the supplier bring you a fresh supply of the same material to try it. Of course, I don't know why you have powder laying around that is 2 years old. Textured finishes can be a bit sensitive in the uncured state, and a powder that is 2 years old should be thrown away. I think that if you get some fresh material, you won't have any pinhole problems. So, hey, don't be so cheap. Go buy some new powder. The industry would appreciate it.
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12/19/2011
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Q: We do a lot of powder coating for a Corvette restorer. We have done a number of old intake manifolds in the past, but recently have had some complaints. Our customer claims that by powder coating these intakes, they are cracking. I can't believe that baking an aluminum intake would cause it to crack. We bake it to 392°F for 10 minutes. I think they must have been cracked before we got them. What do you think? P.L., Export, Pa.
A: On "used" metal, all assurances go out the window. Sometimes well-used aluminum doesn't like the surface tension that occurs between itself and the powder. However, there is no way of knowing what the alloy is, so it's a bit of a gamble. Do you know the location of the cracks? Are they near the exhaust headers? If you have had good luck with very similar pieces, then I would tend to agree that the cracks were already there. So what can you do? I would do a thorough inspection of the parts prior to coating. Use a light if you can to attempt to locate any cracking. Aluminum parts are baked out at higher temperatures than you’re using without problems, so my guess is your suspicions are correct.
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12/13/2011
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Q: I use a variety of powders, including acrylics, polyesters, polyester-urethane hybrids, and so on, in a spray-to-waste batch operation. I use these powders in the same powder booth but with different hoses and careful cleanup between. Cleanup consists of blow-off (and occasionally a water wash down) and vacuuming of the whole booth and wet vacuuming of the floor and path to the oven. The oven is also blown out and vacuumed prior to start-up in the morning. The booth is equipped with a pulsed cartridge filter system. This source of scrap powder is also cleaned up between powder changes. I’ve read about contamination of work between powders but wonder if the practice is feasible with the amount of cleaning we're doing between powder changes. I've had one incident of fish-eyeing that could have been powder contamination or a pretreatment failure. The problem did not recur—yet. L.J.
A: The major problem is between acrylics and polyesters. You’re changing hoses, I assume cleaning or changing hoppers, and you don't reclaim, so you should be OK. Generally speaking, the various polyesters and epoxies tolerate one another, so that shouldn't be an issue. Yes, the fish-eyes could and probably did come from some contamination on the parts that the washer couldn't remove. What I've seen as cross-contamination from incompatible powders has been craters rather than fish-eyes.
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12/6/2011
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Q: I have some problems with the powder we’re using. It has balls. I increase the pressure of the flow and the balls aren’t destroyed. I think the transportation method or the high temperature is the problem. I use powder in barrels of 300 pounds with the fabrication dates on them. The powder is not old. Do you think this is sinterization? M.S., Mexico City, Mexico
A: Well, you’re on the correct path. You just don’t know how to fix it. Powder balls, or agglomeration, come from several sources: packing during shipping; shipping in hot weather with no refrigeration in the truck, especially long distances; moist compressed air; and high humidity. If you’re buying in 300-pound drums, you’re a high-volume user. As such you should invest in a drum unloading device and a sieve. The good thing about this equipment is that it conditions the virgin powder and the reclaim. The purchase of this equipment is imperative. Having said that, if the powder is getting moist in the feed hopper, caused by compressed air that isn't dry enough, or the area around the hopper is very damp, then you have a different problem, and the answer should be obvious. The air dryer should be located next to the booth, and properly maintained. As for the atmosphere, remove the source of the dampness, or put the powder booth in an environmental room.
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11/21/2011
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Q: My company has manufactured welded components for years by using a variety of steel. Our operation includes an in-line blaster and a six-stage iron phosphate system. We use a polyester powder topcoat. We have always had two employees manually reinforce Faraday cage areas before the parts are coated with our 10 automatic guns. I was given a project to reduce the manual reinforcement to one person. We have a very diverse product line with multiple concerns, that is, we have deep corners and pockets on each component. Although I’m early into the project, it appears to be quite a challenge to reduce reinforcement while maintaining our quality level and consistency of mil thickness. I’m certain racking orientation can be changed to standardize a lot of Faraday areas so that they face the operator. Any helpful hints on gun positioning, maximizing coverage, or success stories of other companies that you can provide, will be appreciated. S.H., Johnson City, Tenn.
A: Your charge (excuse the pun, but not intended) is to eliminate bodies, or at least a body, and the alternative probably is the addition of a spray gun or two. At least the gun doesn't need health care or vacation or time off for illness. As you noted, you may have to really rack your brain to see how many parts on the manual reinforcement list can be hung so that one person can do the job. The next thing to consider would be to mount a "stray" gun or two on an independent gun bar, maybe affixed directly to the cabin wall, that would be directed to a weak spot. I've seen some weird looking gun arrangements in booths, and they are invariably for that purpose. Tell the boss that this won't be easy or quick, but it may be possible. Set up a spare gun and controller from spare parts inventory for trial. If that works, use the cost savings to pay for a new gun, or two, or whatever is required. The trick may be, if you don't have gun triggering or part ID, is to figure out how to do this without creating too much film thickness in other areas.
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11/14/2011
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Q: We’re trying to powder coat a gas tank. It’s the type that sits in the bed of a pickup truck and is used to fuel other vehicles. The tank has been sitting for a couple of years, and there is some sludge in the bottom of it. A strong smell of fuel is also coming out of the tank. Is it safe for me to put the tank into my conveyorized infrared oven? T.S.
A: That depends on whether you’re a pyromaniac. Or maybe you just enjoy a good fire. It's a pretty good guess that the tank was used for diesel fuel, which generally doesn't give off fumes that will explode, at least not like gasoline does. Anyway, if the tank held gasoline, and the tank has been setting for a while, the gas fumes would be gone. Absolutely do not put this tank in the cure oven until it is properly cleaned. I hope your insurance carrier doesn't get wind of this. Find a good solvent that will aid in dislodging the "sludge," which I'm sure was initiated by some leftover fuel, and repeatedly flush the tank. By the way, when you get around to it, don't put the filler cap on the filler tube when the tank goes in the oven.
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11/7/2011
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Q: What’s the best method for removing multiple layers of TGIC (triglycidyl isocyanurate) powder coating from plugs and fixtures? I don’t have a separate burn-off oven. Sandblasting seems to take entirely too much time from production. I have experimented with some chemicals, but got poor results. J.H.
A: I must assume that you’re referring to metal plugs and of course metal hangers and hooks. Plastic plugs are another matter, and you can't use plastic hangers. What a brilliant deduction on my part, eh? Removing powder coating from metal can be done in three ways; burn-off, hot sand, or chemical cleaning systems. You might be able to clean small pieces by soaking in something like MEK (methyl ethel ketone), but there are so many precautions that I don't recommend it. And those are the only methods available.
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