• For optimal results, a car should be block-sanded before it is painted.one headlight image by Thomas Czeizinger from Fotolia.com
  
  Block-sand your car's body during a spray-paint job to create a surface for the paint to stick to. Although poor sanding techniques may result in sanding scratches and wavy panels, these techniques are not hard to master. The secret is to change the sandpaper frequently and to keep the surface as clean as possible, while wiping down the car's body often with grease and wax remover.
  
  

Sanding Over Existing Paint

• To save time, some spray painters sand and prime over the car's existing painted surface. When using this method, scuff the old surface before applying new primer and paint, otherwise the paint will start to flake quickly. Use a 500- or 600-grit sand paper, or a fine-grate pad for the best results. Work in all directions to create a rough surface for the paint to stick to. The downside of this technique is that you don't see or fix blemishes such as rust or trapped dirt under the original paint layer.
  
  

Stripping Paint

• You will have the best grasp of the body's condition if you strip all existing paint. Ask a body supply shop to recommend chemicals for your car's paint type. As soon as all the paint is stripped off, start sanding with relatively coarse paper, such as a 200-grit paper. Gradually start using finer paper, working up to 1000 grit.
  
  

Sanding Blocks

• Use an assortment of sanding blocks of different sizes to suit the shape of the car's body. Use the longest block that will fit into the area you are sanding. You can build your own blocks from Rubatex Rubber for specific spots on your car's body. Use a flexible block on the car's roof or other sloping areas where you want as much contact with the surface as possible. Use small blocks for tight spots like door posts and quarter panels. Jefflilly.com recommends using a round foam block backed by an acrylic rod for round shapes. These rods are available from body-shop supply stores.
  
  

Sanding Movements

• Learnautopainting.com suggests that the best technique to follow when sanding is to move the sanding block in all directions. Do not just maneuver back and forth, but go criss-cross, up and down and diagonally to create a coarse surface for the paint to stick to.
  
  

Primer

• Hotrod.com recommends you apply primer to the body at least twice before applying your first layer of paint. Apply spot putty to correct flaws, sand it smooth and cover it with primer before sand-blocking the whole body again. Use a smoother type of sandpaper with each successive blocking session, moving up to 400 grit before the painting stage.
  
  

Guide Coat

• Some auto painters prefer to apply a guide coat before sanding. Spray a light layer of paint in a contrasting color to your primer before sanding to help you show the spots that have been sanded. This guide coat will reveal high and low spots on the car's body. Feel for flaws in the surface by running your fingers over it.