Originally Posted by TOGWT
A surface scratch that will `catch' your fingernail is approximately 0.04 Mil (1.0 µ) deep will usually require wet sanding and the clear coat refinishing Removing a scratch requires removing the layer of paint that contains the defect; you need to level the paint to the lowest point of the scratch. Removing more that 0.5 mil (12µ) of clear coat will cause premature paint film failure as UV protection percolates to the top of the clear coat. Check paint film thickness with a Paint Thickness Meter (PTG) before you attempt to remove
As you go over a deep scratch, the abrasives round off the edges of the high spots of the scratch. The result is a shallower scratch (when no full correction can be made) rounded edges don’t reflect light the same way a sharp edge will and is therefore less noticeable.
It may be necessary to carry out some localized wet sanding to facilitate full removal of any deep scratches, once again, paint thickness must be checked, and if the paint is too thin wet sanding should not be considered
I've done it. the result is much better but it is exponentially more work. And you need practice. If your car gets keyed this is a way to go. For a nick, well Do you wan't to prevent rust and further paint damage, or do you want perfect?
Either way go get an old fender and play! You feel all accomplished and stuff!