Assorted sandpaper
Sanding is the most labor-intensive step when it comes to painting a car's surface. Without sanding, a paint job wouldn't look smooth and glossy. When sanding a car, you can't just use any sandpaper you find laying around. Sandpaper with a low grit number is too harsh and can leave deep scratches. High-grit sandpaper won't leave scratches, but it can't sand body filler or primer effectively. Sanding is a process that takes skill and experience, but learning some basics can result in smoother finishes no matter what type of sanding you are doing.
Instructions
1. Sand using the low-number sandpaper first. For sanding body filler, use 80-grit sandpaper and move up to 180, 220 and higher grades. Start with nothing lower than 400-grit sandpaper for sanding primer or for removing the top layers of clear coat only. Use only 800-grit or higher sandpaper for wet-sanding projects.
2. Place sandpaper on a sanding block for most projects, including sanding body filler and primer. A sanding block helps keep the sandpaper even and flat on the surface, unlike your hand, which can press harder in some areas. Long sanding blocks are best for most areas of a vehicle. Short sanding blocks work well around corners and along body lines, as long as you keep them flat on the surface.
3. Keep the flat surface of the sanding block flat against the vehicle. Use the long sides of the sanding block as the leading edges instead of the short sides, to avoid forming grooves on the surface. Use an X-shaped pattern when sanding to make sure the surface is sanded evenly and to prevent waves on the surface. Don't turn the block on its side or use the edge for sanding, since it can quickly cut a noticeable indention or line in the filler or primer.
4. Move the sanding block along the vehicle from front to back, not from top to bottom. Even though you use an X pattern, the general direction is always horizontal, not vertical.
5. Clean the surface by blowing it off with an air compressor, rinsing it with water or at least wiping it with a towel between sanding stages. A clean surface shows the progress you've made with the sandpaper and can show any places that were missed.
6. Soak the sandpaper in a bucket of water when using 400-grit or higher paper. The fine-grit sandpaper sands the surface, but build-up can form under the paper, which can cause deep scratches in the surface of the car. Keeping the sandpaper soaking wet prevents this from happening.
