And more surface area means more places for the new coating to cling to. The surface area to be painted expands by the size of the sand scratches (measure up one scratch, down the next). In my experience, even a light-scuff sanding will help adhesion markedly. Sanding effectively doubles the surface area, thus providing twice the opportunity for the new coating to adhere to the old. While this can be a substantial time saver, I’m an old school guy (not to mention just plain old!) so I’m always happy to have the painter sand the part. Several OEM undercoats don’t have to be sanded under perfect circumstances. As many of your paint reps will tell you, it’s now possible to simply wipe clean and begin spraying on certain OEM parts under certain conditions. Next, let me say that sanding any part prior to refinishing is a good idea. If your dual-action, random-orbit sander is going so fast that it sounds like a jet engine whine, it’s likely that you’ll have problems painting plastics. Plastic parts, however, absorb the heat and can soften or even re-flow under power sanding. Metal parts also get warm as sanding friction heats them up, but metal tends to reflect the heat back out. Sanding friction causes heat, and plastic parts act like a heat sink. Let’s start by talking about sanding plastic components. Whether thermoplastic or thermoset, today’s parts can be treated uniformly. I’m pleased to report that to successfully paint plastic automobile parts today, it’s no longer necessary to name the plastic first. Some identification tests included floating a chunk of the unknown material in water. Over time, painters were expected to burn a sliver of the mystery plastic and name it based on the color of the flame or the smell of the smoke. When the identifier was missing, other methods of identification were necessary. And auto painters have suffered through a lot of grief trying to identify the type of plastic based on a two- or three-letter identifier stamped (seldom) on the back of the part. It’s been a long road from the first flexible bumper cover on the 1967 Pontiac GTO to the latest body cladding on the 2004 Pontiac Aztec.
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