Should I start my sander on or off the wood?
When using a random-orbit sander, should I power it up and then place it on the workpiece, or switch it on after it's on the surface to be sanded? I've seen people do it both ways.
—Robert Mobley, Van Nuys, Calif.
For best results, Robert, start with the sander in hand and flat on the workpiece; then turn it on. When done, lift it off before powering down. An easy way to remember the rule: "Start on, stop off."
Here's why: A random-orbit sander's pad begins spinning when you engage the motor, but it doesn't oscillate in small orbits until you apply downforce. (The weight of the sander and your hand resting on it provide sufficient force—don't push down.) But if the disc is already spinning at full speed when the abrasive contacts the wood and begins its oscillating action, the sander can abruptly grab, skip, or jump, especially with 120-grit or coarser sanding discs, scratching or gouging the workpiece. That then requires more extensive sanding or scraping to remove. Starting the sander on the workpiece keeps the spin under control.
Stopping the sander while it's in contact with the wood would be like slamming on a car's brakes on pavement, except in this case the skid marks would be pigtail sanding swirls on your workpiece.