Issue link: http://www.e-digitaleditions.com/i/653106
POWDER COATING, March 2016 33 A You have little control over or- ange peel, but you have a lot of control over fisheyes. Orange peel is an appearance effect that comes with the territory in powder coating, so to speak. This effect can be minimized by using a thin film or fine-grind mate- rial. I suspect that your powder supplier will discourage you from using finely ground material because of handling problems. If you have a material that is producing an objectionable orange peel, talk with your supplier or coater for help, but don't expect miracles. Ap- plication technique can also produce or- ange peel, but again don't expect big changes by correcting your method. Fisheyes, on the other hand, are the re- sult of a contamination problem and are usually associated with liquid paint. When they occur in powder systems, it's typically because oil or some petro- leum product has gotten into the pow- der, usually through the compressed air system. A fisheye is just that. It's a raised, clear bubble in the cured or un- cured state. Don't confuse fisheyes with craters, which are more common in powder coatings. Craters are also caused by con- taminants on the substrate surface, only the crater will be to the base metal. Crater depressions, which are a dip that doesn't go all the way to the base metal, are usually caused by a problem in the powder material and are hardly ever corrected. Most often, your powder supplier will need to replace this conta- minated product. I've seen old powder have craters, which is a shelf-life issue. But don't blame that on your supplier, unless you are deliberately buying pow- der at a cut rate. So, part of your prob- lem is fixable; part of it is just the way it is! —G.T. Figuring out a machining process Q If we premachine a part, then do a pretreatment or phos- phating process, and then powder coat it two to three days after final machining, is that an effective process? Or do we have to repeat the phosphating process? We can't powder coat or pretreat our part's internal sur- face, so we are only processing the outer surface. Please help. U.P, Gurgaon, Haryana, India A Pretreating your parts, machin- ing it, and then powder coating it will not work, because the ma- chining process will deposit soils back onto the surface causing powder adhe- sion problems. If you have critical ma- chined areas where pretreatment or coating cannot be allowed to cover, then the best method is pretreating and coating your parts first and then post machining these surfaces. The coating will be removed during the machining process and you will not have to deal with masking the surfaces either. Most powder coatings can withstand the fix- tures and holding systems used to posi- tion your parts during machining. However, if you do get tooling marks, changing to a more durable coating or using plastic protection on the tooling can eliminate these issues. —N.L. Repairing a part finish in situ Q I am a building surveyor and have a project in which some powder-coated balustrades have been damaged down to the bare metal. Can the finish be repaired in situ, and how can it be done? I would be most grateful for any advice you can offer. J.H., Chichester, West Sussex, UK A You have two options for the re- pair of the damaged goods. Your choice will be heavily influenced by the situation with the parts; that is, whether or not they are installed in their final resting place. The options are to re- coat with powder or to touch up with a color-matched liquid. Now, there are several qualifiers to re- coating with powder. Should that be the choice, you would have to recoat the entire part. A powder coated part is dif- ficult to spot repair because the second coat leaves a halo at the repaired area, creating an unacceptable appearance. Several other factors also enter the pic- ture. A second coat will accentuate the orange-peel appearance, and finite de- tails, such as embossments, will be cam- ouflaged by the heavy powder film thickness. Touching up with a liquid is a perfectly normal and acceptable way to repair a powder coated part. Be certain that the repair area is well-sanded or buffed and that the touch-up is a good quality coat- ing. You may want to use an automo- tive repair procedure, which includes a liquid prep application before applying the final coating. This would give a bit of metal protection to any exposed sub- strate. Is the spot repair area as good as the original coating? Nope, it never is, regardless of the chemistry of the origi- nal coating, but sometimes you don't have a choice. —G.T. Asking too much from your application equipment Q I'm having some problems while applying a second coat of translucent blue. I'm trying to do blue veining. The first coat looks great, but the second looks like crap. The problem I'm having is that when I apply the second coat, the powder won't stick in some places, such as the corners and crevices, or it will blow off while I'm trying to apply it. Sometimes, both of these things happen. In some places, the blue looks purple and in other places, it looks kind of green. I'm using a do-it-yourself gun that comes with powder. I don't know if the prob- lems have to do with the gun, the pow- der, me, or the ground, which I have heard can cause problems like this. I'm using coat hangers to hang the parts. Thanks for your time and help. W.F., Crawfordsville, Ind. A Where do I start? Do-it-yourself application equipment is in- tended for the hobbyist and isn't meant to be used in a very sophisticated way, so don't set your sights too high. I don't know where the powder comes from for the gun, and I really can't com- ment on its application capabilities or performance standards. Using a coat hanger as a hook is okay. If it's a painted hanger (most of them are), make sure you remove the coating in the area of the contact point with the part and to the ground point. Corners and similar areas, such as crevices, create a Faraday cage effect when electrostati- cally applied paint is used. Here's the problem: The charge goes to the nearest