Powder Coating And Polishing Basics

For as long as humans have been working with metal, we have been polishing it for the simple pleasure that the sheen brings to the viewer. Naturally, we want our motorcycles to be shiny too. When compared with mankind's longstanding relationship with polished metals, powder coating is a relative newcomer--but one that's steadily gaining popularity in both the manufacturing and customizing aspects of motorcycling. While one process is old-fashioned and brutish, requiring hard physical labor, and the other is high-tech, employing electricity, heat, and epoxies, both can be used individually to create stunning finishes on motorcycles. Combining the two produces a synergistic relationship that can become the crowning touch of a custom project.

Powder Coating

Powder coating is a dry finishing process in which a powder, consisting of finely ground particles of pigment and resin, is given an electrostatic charge and sprayed on the part to be coated. The charge holds the powder to the part until the curing oven melts and fuses the powder to a uniform coating. Since the powdered paint doesn't contain the volatile organic compounds (VOCs) found in most liquid paints, powder coating is an environmentally friendly process that provides a durable finish. Scott Andrews, owner of Andrews Powder Coating, Inc., understands what customizers want from a powdercoater. When we approached him for this project, he suggested we combine polishing and powder coating, and even recommended that we have all the venting holes in the discs powder coated, a process which requires that the whole disc be masked and each hole uncovered to accept the powder.

When parts arrive at Andrews Powder Coating, the process starts with surface preparation. Andrews sandblasts parts with aluminum oxide to make sure any previous coatings are completely removed. Aluminum oxide etches the surface of the metal, providing "traction" for the powder to mechanically bond while making the powder coat more durable. If the parts were previously powder coated, he will have them chemically stripped before the sandblasting to be doubly sure that the surface is ready for the powder coat. Next, the parts get prebaked at 400 degrees for about 20 minutes. Andrews feels that the prebaking of parts is one of the characteristics of a top-notch powder coater. The prebake temperature is approximately 20 degrees hotter than the actual powder-coat baking temperatures so it bakes out any impurities that may be lurking within the metal's pores (a process called "outgassing"). If these impurities are not cooked out, they could bubble up under the finished powder coat, making it possible for a small chip to cause whole sections of the coating to fall off.

After the initial surface preparation, Andrews sends the parts for polishing to Derek Stankovich at Distinctive Metal Polishing. Upon their return from Distinctive, the parts receive a solvent bath and another prebake. Then the polished portions of the part, and any other sections that should not be powder coated (such as the piston holes in the calipers), are carefully masked with high-temperature-resistant tape. The masked parts make their way to the paint booth where they are hung on electrically grounded racks. The powder is given a 30,000- to 90,000-volt charge and is sprayed onto the part, where it adheres to material much as dust clings to a TV screen. One advantage of powder coating over painting is that if the person applying the powder is not happy with the thickness or look of the powder, he can brush or blow it off with compressed air and reapply. Also, areas of the part that are too time-consuming to mask may simply be wiped off with a finger. However, unlike liquid paints, powder-coated candy colors, such as the candy blue used on our Valkyrie's wheels and calipers, require multiple coats of powder. Candy colors require that a layer of silver be baked on before a translucent color is applied over the silver, making candy colors cost more than plain colors. Andrews says high-end powder coaters will always wait for the parts to cool down to the ambient temperature before the second color's application. Spraying the powder on hot parts results in uneven coverage and an inferior finish.

The oven is where the action takes place, and Andrews' oven is a sight to behold. Large enough to hold an entire car frame, the 24-foot-deep oven once baked an 8-foot round, 14-foot tall bird cage. Since the metal of the part must become hot enough to melt the powder for optimum adhesion, the time parts spend in the oven is determined by their size and weight. Calipers will bake for 20 minutes at 380 degrees during the application of the base color, while wheels need to roast for a full 45 minutes. Curing the final coat takes longer (40 minutes for a caliper and almost an hour for a wheel) to assure that the powders have fully melted and bonded to the surface of the part. Once the parts have cooled, the masking tape can be removed from the finished part.

Polishing

Heat plays an important but different role in polishing. Instead of using heat to bond a substance to the metal, the heat of friction in polishing--reaching temperatures as high as 400 degrees--causes the surface of the metal to fuse together, closing its pores. Polishing is, in its most basic sense, the progressive elimination of scratches through the use of finer and finer abrasives. The sheen comes from making the scratches in the metal so fine that they are invisible to the naked eye. In the final stages of polishing, rouges (compounds of grits, lubricants, and metals that help to bring out the metal's natural color highlights), not sandpapers, provide the ultra-fine abrasives. Although brute force plays the primary role in polishing, a polisher's finesse with the detail work demarcates the class polishers from the masses.

When a part arrives at Distinctive Metal Polishing, Stankovich sands the part to remove the large imperfections (including brand names, part numbers, and casting seams) from the surface. Large faces of the part get the treatment from a belt sander shod with 220-grit sandpaper. Smaller sections of the part require a sanding wheel, a hand-held belt sander, or a Dremel, depending on the intricacy required. Detail work is time-consuming, but Stankovich says the use of hand-held tools separates a show-quality polishing job from an average one. He estimates that about 80 percent of the time he spends on polishing motorcycle parts is consumed by the Dremel. For example, nine-spoke cast Harley wheels and eleven-spoke A.C.E. Tourer wheels require around 40 hours of labor, simply because of the tight spaces in the spokes. After the 220-grit sanding, a 400-grit sanding takes out sanding scratches, continuing the process of eliminating scratches.

Once the surface has been sanded, the sisal wheel, covered with a nylon and denim-like material, begins the actual polishing. Brown or black rouge, equivalent to 1200- to 1400-grit, begins to bring out the metal's luster. The final step in polishing is running the metal under a cotton wheel with green rouge, which, at 2000-grit, is a true polishing rouge. The previous rouges just took out scratches. The chromium in the green rouge bonds with the metal, giving the polished metal its characteristic sheen. Since chromium is transparent, the individual character of the metal seems to shine from within the part itself.