When painting cars, it is common to spray on a coloured base coat and follow with a protective clear-coat. This makes it possible to buff the surface coat without damage to the colour. This is a technique that has other applications around the house, such as appliance and furniture refinishing, as well as wall faux-finishing. The colour and spreadability of the base coat is more important than the durability of the paint you use. It is important, though, to ensure that the clear coat is compatible with it.
Prepare the surface for painting by sanding holes or dents with an orbital sander. Use a medium-grit paper to remove rust from metal surfaces, and then spread rust-preventive liquid on the rusty areas with a small paintbrush. Fill all holes, dents and rusty areas with wood or metal filler, and sand the filler flat with the sander.
Sand the entire surface with the orbital sander again using progressively finer-grit sandpapers to smooth it down. Finally, sand the surface by hand with a very fine-grit sandpaper to remove marks left by the orbital sander. For a wooden surface, use 220-grit sandpaper for this final pass, and for a metal surface, 400-grit paper.
Fill the cup of a siphon spray gun with wood or metal primer and screw the cup onto the gun. Plug the gun into a compressor and test the spray pattern on a scrap piece of wood or metal. You want a spray pattern that fans out to a width of about 6 inches from a distance of 6 inches. Adjust the air pressure and tighten or loosen the nozzle until you achieve this. If the spray is too thin or chunky, add a little thinner or reducer to the primer.
Spray the entire surface by holding the gun 6 inches away and spraying in a steady up-down or back-and-forth motion. Keep the gun a uniform distance from the surface as you spray and overlap about half of the spray pattern on each pass. When you are finished, you should leave a wet coat without drips, sags or voids. Then immediately spray the surface again, moving the gun in the perpendicular direction.
Let the primer dry for the time specified on the container, then sand it lightly by hand with fine sandpaper. If the surface is made of wood, use 220-grit paper and if it is made of metal, use 400-grit paper. If there are any drips, be sure they are completely dry before you sand them flat.
Clean the cup and tip of the gun thoroughly with thinner, and spray a little thinner through it. Then fill the cup with the base coat, adjust the spray pattern and thin the paint as needed. Spray the surface with the base coat in the same way as you sprayed the primer but spread a slightly thinner coat. Most base coats dry more slowly than primer, and the likelihood of drips is greater.
Let the base coat dry for the specified time, then respray. You probably won't need more than two coats, but if the coverage is thin, spray a third coat. Lightly sand the last coat in preparation for the clear coat.
Automotive refinishers add a stabiliser to the base coat to cause it to spread evenly over the surface and create a uniform colour. Check the reducer you use to see if this stabiliser is present and reduce the paint to the manufacturer's recommendation, or you can add a stabiliser yourself.
If you use a catalysed, two-part base coat, don't let it sit in the gun for too long, or it will harden there. The fumes from most paints are harmful. Wear a respirator while spraying and keep the workspace well ventilated.