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pfonline.com/experts 40 NOVEMBER 2016 — PFonline.com THE VOICE OF FINISHING 80 YEARS POWDER COATING CLINIC RODGER TALBERT powdercoating@pfonline.com Sanding for Repair Q. We sand aluminum die-cast parts that have minor defects in the powder coating before recoating them. The small areas of dirt, metal blemishes, fibers or similar particles are sanded using 120-grit sandpaper on an orbital sander. After recoating the parts, we often find more little specks and other defects and lines from the sanding disk in the coating. My question is, does this happen because of the sand- paper we are using, or because we sand all the way to the aluminum, exposing the surface and causing it to outgas? A. Sanding is necessary for removal of simple particles, scratches and similar defects but care must be taken to avoid a repeat of the defect. You may be getting some outgassing, but it is most likely particles left behind after sanding or cut marks made from the sanding operation. For starters, work on the grit size. The 120-grit paper is fine but it may not be fine enough to avoid lines, especially if you use a motor- driven method. You may need to follow up with a 240-grit to get it smooth. Also, make sure to keep the rotational speed of the device slow enough to control. It is easier to cut deeper at higher RPM. Think of it as finessing, not cutting. Work with your operators to understand the process and the potential pitfalls of too much pressure. Make sure that all sanding dust is removed using clean, dry compressed air or run the parts through a deionized or reverse osmosis water rinse. Better sanding technique, finer grit and more careful removal of dust are the keys. Measuring Film Thickness before Cure Q. How can I tell if the powder film build is thick enough before I cure the powder coating? We coat a lot of square footage and we need to control our cost. We often put on too much powder to avoid light coating. I need to know what my uncured film thickness is. A. I suggest you start by running regular trials to establish operating parameters that will provide a specific film build. A spray gun or guns operating at a specific output and position will provide coating thickness that is reason- ably predictable. If you use standard operating settings and the equipment maintenance is good, the film build can be predicted within an acceptable range for most coaters. Some coaters use more sophisticated controls to meter the output of powder in grams per minute for more precise flow control and film build control. There are also a couple of thickness gages available that can measure film thick- ness before curing. In installations where a lot of powder is used a savings in film build of 1 mil could provide an excel- lent payback for a dry film thickness gage. Discoloration of Metal-Flake in Powder Q. We have a metal-flake silver powder color that we use regularly. If the surface is abraded with a Taber wheel—or even a pencil eraser—it darkens significantly in the abraded area. We even have this problem when our parts scrape against cardboard packaging. Our supplier suggested we switch from an unbonded leafing aluminum to a more expensive bonded product. We tried the bonded leafing aluminum, but we still have the same problem. Certain types of friction create this effect, seemingly those that perhaps have a higher coefficient of friction. Do they remove the aluminum flake from the surface? Can you explain this problem and tell us why it happens with some abrasives and not with others? What can we do to stop this? A. What is happening is that the abrasive is removing any resin that may be laying over the aluminum flake and abrading the surface of the aluminum so that it does not reflect light. The flake shows a flat, gray color where it is abraded. Some abrasives will polish the surface and not cause the darkened look. The rubber eraser probably contains some high-silica pumice or similar abrasive product that scuffs the soft aluminum surface and creates the darker color. The Taber abrader cuts the aluminum surface and breaks up light reflection. A soft substance that lacks sharper edges will not cut and may even polish the aluminum flake. The issue is that you do not have enough hard resin over the flake to protect it for the abrasion. Chances are, the silver color has a fairly high amount of aluminum content. Bonding may help a little, but it will not provide reliable resin thickness over all of the aluminum flakes. I would try some different colors, possibly from different suppliers and with less aluminum content. If changing color is not an option, you will defi- nitely have to apply a clear coat over the silver finish to protect it from abrasion. Masking for Logo Design Q. We are powder coating stainless steel water bottles for our company. For our logo, we would like to use a vinyl sticker or decal before powder coating. After the coating is applied, we want to peel the vinyl off to achieve a stainless steel logo effect, so we need some kind of temperature- resistant vinyl. Any suggestions? A. You could use high-temperature polyester tape. It will take up to 425°F in the oven, and it can be made in die-cut forms for your logo. You can find it from supply sources that can be researched through PFonline.com by searching "masking supplies" or "high-temperature masking tape." There is one caution that is always offered