Painting a vehicle with base coat/clear coat paint presents some challenges when compared to single-stage paint, but with careful preparation, base/clear provides a superior finish to single stage in that it is more resistant to chipping when struck with debris from the road. It also provides better ultraviolet protection and protection from chemicals such as those in acid rain.
Surface Preparation
Surface prep for urethane base/clear is critical to the appearance of the final product. The surface must be sanded with nothing coarser than 400-grit sandpaper. The urethane will fill to that level and smooth out, but won't on anything with a lower grit number. Additionally, if the base/clear is being sprayed over existing paint, scuff the surface with a gray automotive scratch pad before painting. You have to remove all of the gloss from the original surface or the paint won't adhere to the vehicle. Finally, the surface must be absolutely clean to prevent what are called "fish eyes" in the paint. This is done using wax and grease remover both before and after the original paint is scuffed.
Spraying the Base and Clear
The base coat for the base/clear paint combination should be sprayed in two to three coats with the first coat being a light tack coat. Using more than three coats will result in a finish that is too deep. When the finish is too deep, the paint job loses its flexibility and is susceptible to cracking. The same is true when spraying the clear coat. The first coat should be a light tack coat followed by two medium-heavy coats. This is enough to protect the surface and still give you room to sand down the dried clear coat to achieve a perfectly smooth finish.
Finish Work
After the base/clear has cured, or dried, finishing the paint job is done by wet sanding the paint with between 1000- and 2000-grit wet/dry sandpaper wrapped around a polyurethane sanding block. Ideally, you will be removing as much as one third of the orange peel from the surface of the paint job. Once the surface has been sanded, use an orbital buffer with a foam buffing pad set to its lowest speed to polish the clear coat. Urethane paint dries hard, but it isn't impervious. Using the buffer's lowest speed prevents the machine from breaking through the clear top coat to the urethane base coat below.
Safety Gear
Whenever you paint with urethane base/clear paint, you need to be aware of the dangers inherent in breathing in the fumes from both the paint and the clear top coat. While minor exposure is not very harmful, technicians who paint with urethane base/clear over time without the aid of protective gear will experience respiratory problems and severe discomfort. Always wear a protective breathing mask as well as a long-sleeved shirt or paint suit when spraying urethane base/clear paint.
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