Sanding Basics - How to Sand
Here are some basic sanding techniques that can help you obtain a premium finish.
The objective in sanding is to remove all deeper scratches and leave fine scratches from the fine sanding operation. The fine scratches allow the paint to adhere better to the surface.
Using a coarse grade of sandpaper, go over areas where marks from shaping the body filler may remain. After you have removed all deep marks, switch to medium-grade sand paper and remove all marks from coarse sanding. End with a fine grit paper.
It is important to remove all of the marks from sanding with the previous grade paper before moving on to the next grade. Fine-grade sandpaper will not remove all of the marks, so begin with a coarse grade on the initial go-around when working with body filler.
If you are sanding stable paint work that needs a better shine, it is OK to start with a medium grade sandpaper. If you are sanding between coats of paint or lacquer, use a fine grade of sandpaper.
Use the "flats" of your fingers, as opposed to your finger tips, when sanding concave or small areas. This way pressure will be distributed evenly over the area. Keep your fingers at right angles to the direction of rubbing and your wrist close to the panel. This prevents the "grooving" that using individual fingers will cause. Always sand in straight lines: sanding in circles will show lines, whereas sanding in straight lines will not.
To remove road tar or grime from the surface, use mineral spirits before you begin sanding. Attempting to remove grime or road tar with sandpaper will only drive it deeper into the surface. Remove all such contamination completely before you begin sanding.
It is OK to use a sanding block on slightly convex and larger level surfaces.
You are going to use a technique called "feather edging" to blend painted areas. Use this method after you have applied new paint after a repair, after you have completed repairing rust damage, or where the paint in an area is simply too thin. This means you are going to sand so that there is no noticeable ridge where the new paint begins. Sand over a wide area, feathering the edges gently to prevent them from crumbling.
Primer, sanded paint, and body filler all have a bad habit of hiding faults. These faults, however, will show through after you apply your coats of paint. You can more quickly and easily spot these faults by using a "guide coat" of cellulose lacquer, either from a spray can or heavily thinned from your spray gun. This provides a nearly transparent glossy coat that shows you the faults. Avoid using shades of yellow or red for the guide coat because these colors will most likely show through the layers of paint you apply subsequently.