Orange Peel Repeal

A top-notch factory paint job is de rigeuer in today's automotive sector - and, quite fortuitously, painting technology has kept pace with the expectations of customers so that everything from Focus to Ferrari boasts a wet, mirror finish that not all that long ago required meticulous hand-finishing.

The brilliant, mirror-like shine of today's finishes mainly is due to ongoing advancements in high-gloss coatings and painting techniques. But the high level of basic factory painting to which customers now are accustomed makes it more important than ever to avoid the classic quality nemesis that plagues painted finishes - "orange peel."

Orange peel can best be described as a wavy, slightly lumpy, light-and-dark pattern resembling - what else - the skin of an orange. You know it when you see it: the magic just isn't there.

Bill Bastian, transportation sales manager with BYK-Gardner USA of Columbia, MD, since 1931 a maker of precision color measurement instruments, says the need to control orange peel is by no means restricted to light-duty vehicles. Quality paint finishes now are crucial for motor homes and luxury buses, and the issue is gaining attention in the pleasure boat and aircraft markets.

"There was a time when a truly high-quality car finish meant several coats of hand-rubbed lacquer," Mr. Bastian explains. "Today's car buyer expects a flawless finish right off the production line, which requires sophisticated painting systems and precision quality controls."

The "wetness" of today's shiny paint jobs isn't the result of paint alone. A high-quality finish depends on what coatings experts call "distinctness of image." Although paint makers have succeeded in developing high-gloss liquid coatings, there still are underlying problems in controlling waviness - orange peel - in the final appearance of finishes.

"Each layer of paint, - E-coat (adhesion promoter), primer, topcoat and clearcoat - must have a smooth, lustrous appearance as well as the requisite physical and chemical properties," Mr. Bastian explains. "This is not easily achieved, since vehicle parts have varying contours, and the sheetmetal from which they're made has an uneven texture from the stamping process."

It's a given that you can't let a vehicle out the door with a substandard paint job. But one of the crucial controlling factors in detecting and measuring the orange peel "look" is the very fact that orange peel largely is a visual, not a tactile, phenomenon. Until now, quantifying the visual look of orange peel has been difficult.

Orange peel traditionally was visually evaluated by comparing samples with varying degrees of orange peel. Or by profilometry, a laboratory process that measures the texture of the painted surface.

BYK-Gardner believed that system could be improved. It recently conducted studies on the effects of substrate roughness and paint flow and viscosity on appearance. A vital tool was the company's Wave-Scan Plus, a portable, hand-held meter designed specifically to evaluate orange peel by taking laser-optical measurements on the finished surface. It's the first precision instrument to "see" orange peel in the same manner as the human eye - outmoding the old method of human evaluation of orange-peel test samples.

The BYK-Gardner orange-peel studies evaluated three different paint systems and substrates with textures, using test panels subjected to different paint processes and baking positions. Then the Wave-Scan Plus laser light took a measurement every 0.08 mm for scan lengths of 3.9 ins. (10 cm) on the painted surfaces, resulting in 1,250 measuring points per scan. Measurements of waviness were automatically correlated in mathematical terms to other industry-known, visual measurement scales, including tension value and orange peel standard ranking.

The process also provided an indirect measure of orange peel-influencing factors such as substrate roughness, flow/leveling properties of paints and process parameters.

The test concluded that changes in short-term waviness - the condition that causes the most noticeable orange peel - are caused by sheetmetal roughness and the use of a clear topcoat. The less noticeable long-term waviness is affected mainly by baking position. The test results also indicated that there is a need to measure for orange peel on a continuous basis.

The Wave-Scan Plus helps to avoid orange peel problems in two ways. First, it detects and measures excessive orange peel in final vehicle finishes. But equally important, the Wave-Scan Plus collects data that can be transferred to a PC to perform statistical and analytical evaluations. This enables the optimizing of quality control systems for the prevention of losses through avoidable rework, scrapped parts and wasted paint.

The Wave-Scan Plus already is in wide use among automakers, including manufacturers of European prestige models such as Mercedes-Benz, BMW AG, Jaguar Cars and Rolls-Royce plc. Mr. Bastian says the Wave-Scan Plus is employed in the luxury bus industry, too, and is used by Boeing and Cessna for evaluating aircraft finishes.

Quantifying the visual quality of paint finishes isn't getting any easier.

For example, there's a fundamental new complication: Automakers are shifting to powder-borne paint in order to comply with the Environmental Protection Agency's year-2003 ban on the volatile organic compounds, or VOCs, found in liquid paints.

Powder paints differ in rheology (deformation and flow) from liquid paints. And most automakers don't yet have benefit of extensive experience in applying powder paint to car finishes.

To get that seasoning, DaimlerChrysler AG, Ford Motor Co. and General Motors Corp. are working together in the United States Council for Automotive Research (USCAR) Low Emissions Paint Consortium (LEPC) to develop application techniques and finish standards for powder paints.