Contrast paint the primer/surfacer
Before you sand, apply a light mist of black lacquer from a spray can over the repaired area. This contrast coating will reveal imperfections that will disappear as you sand. Make sure there is adequate ventilation, and also make sure that pilot lights and other flame sources in the general area are turned off.
Prep for Paint
To sand the primer, use a rubber sanding block and 400-grit waterproof sand paper. The trick is to sand lightly while frequently dipping the paper in a bucket of water to keep it clean. When all of the black contrast coat has disappeared, the repaired area is finished. Then clean and prepare the surface surrounding the repair to accept paint then wipe the entire panel with a solvent-based wax remover. Now, wetsand the area again to be painted with 600-grit waterproof sandpaper. If you're going to blend the paint into the original paint, make sure to sand an area larger than the area where you will be applying the paint. Thoroughly rinse the area, let it dry and then wipe again with wax remover. Mask off adjacent panels, then cover nearby wheels and major portions of the car with painter's plastic sheeting to shield it from overspray.
Mix the basecoat with the appropriate amount and type of solvent according to the maker's instructions. Before spraying, gently wipe the area to be painted with a tack rag, then spray the panel or repaired area. If you are blending, apply only enough basecoat to cover the repaired area, then taper off as you move away from the repair. After the basecoat has dried thoroughly, you'll notice its finish is quite dull. This is so the clearcoat will adhere tightly. Do not sand the basecoat before applying the clearcoat. After mixing the clearcoat with hardener, according to the manufacturer's instructions, apply a medium wet coat and let it flash dry 5 minutes or so before applying a second coat. After letting the clearcoat dry for an hour, remove all masking tape. Then wait one day before polishing.
Chances are the finish already looks pretty good. But to make it flawless, it pays to color-sand and polish the entire repair. Use 1500-grit sandpaper to flatten the finish, followed by rubbing compound to remove the scratches left by the sandpaper. Finish up with polishing compound to make the finish flawless. After polishing, wait one month to apply wax.