There is one rule with block sanding that has pretty much no exception. This rule is the direction a sanding block is used; more importantly, how it should never be used: in a straight line. Always move the sanding block at an angle, or hold it at an angle.
When sanding in an open panel, this is almost never thought about. But, when it comes time to sand up to body lines, or in other tight areas, this is crucial. Grooves and/or under cut areas near the body line can easily happen from the sanding block moving in a straight line. This is also important to remember when sanding off the edge of a panel; especially when wet sanding clear coat.
The above diagram shows what not to do. Holding the block straight, and moving it in a straight line, will sand groves in whatever you are sanding, and fast. This is true when sanding body filler, primer, or clear coat. The two corners of the block should never be going in a straight line.
Above shows how you should sand. Sometimes, you have to sand in a straight line. When you do, be very sure to hold the sanding block at an angle. You still can’t over do this, but at least you won’t sand grooves in the material. The corners and side of the block won’t be running in a straight line this way.
Lastly, you can hold the block straight, or parallel with the body line, as long as you are moving it at an angle. Typically, it’s a combination of these two sanding methods that are required to sand up to these body lines, which usually includes holding the block at an angle and sanding at an angle.