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70mm 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 in- side of the box first, mask the opening, and then powder coat the outside of the box. A word of cau- tion: 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 discol- oring, 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 irregu- lar 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, clean- ing the interior, then placing a mask over the outside while spraying the interior. I assure you all of these methods will re- quire some experimentation. —G.T. Powder clumping in a fluidized bed Q We manufacture aerospace products. One of our product lines is NiCad (nickel cad- mium) batteries for aircraft. Our battery cans are made of cold-rolled or stainless steel. They're preheated and dipped in a fluidized bed. We hold the film thickness between 10 mils and 15 mils. Our brown vat has an occasional problem with pow- der clumping. This isn't a problem with the other colors we apply. Is this a mois- ture or humidity problem even though our compressed air is triple-filtered? And is there anything we can do to prevent this from happening? J.M., Waco, Tex. A It could be that the brown pow- der is a slightly finer grind, which will lump as though it's damp. Your description certainly sounds like a moisture problem. Make sure the air lines to the hopper aren't in a position to pick up condensation even though you're triple-filtering the air. You could try mechanical agitation. Some compa- nies do this as an auxiliary method of fluidization for several reasons, one being it helps break up agglomeration. You can use an air-driven motor and shaft with a prop or wheel on the end. Stick the shaft in through the inspection opening, with the wheel about 4 inches from the fluid plate. See if that helps. —G.T. PC Editor's note For further reading on the problems discussed in this column, visit Powder Coating magazine's website at www.pcoating.com and search the Arti- cle Index by subject category. All articles listed in the archive are available for free download to registered users. George R. Trigg is presi- dent of GRT Engineer- ing, Prospect, Ohio. He has been involved in the powder coating industry for more than 40 years. He holds a BSBA degree from Muskingum College, New Concord, Ohio. Nick Liberto is presi- dent of Powder Coat- ing Consultants (PCC), a division of Ninan, Inc., 1529 Lau- rel Ave., Bridgeport, CT 06604. Estab- lished in 1988, PCC is an independent engineering firm special- izing in the use of powder coating tech- nology. Nick has more than 3 decades of experience in the powder coating indus- try and is a member of many industry associations, including the Application Equipment Technical Committee of the Powder Coating Institute. A registered professional engineer in Connecticut, he holds a Bachelor of Science degree in mechanical engineering with a minor in physics. He can be contacted at 203/366- 7244; email pcc@powdercoat.com; web- site www.powdercc.com. POWDER COATING, February 2018 33