When the belt misbehaves.

A sanding belt that wanders off the rollers, snaps or stops cutting is rarely a mystery. The belt, its tracking, its tension and the way it has been stored explain most problems. Work through the likely causes below; your manual has the settings for your sander.

The belt runs off to one side.

• Tracking. Run the sander briefly and turn the tracking knob a little at a time until the belt stays centred on the rollers. Adjust again after fitting each new belt.
• The wrong size. A belt that is a few millimetres off in length or width may never track properly. Check the size documented for your sander.
• A crooked joint. If one belt wanders but others track, the joint may be out of square. Swap the belt.
• Worn parts. Grooved rollers, a worn platen or a damaged cork pad make belts wander. They are replaceable parts on most sanders.

The belt breaks at the joint.

• Running backwards. Many joints are made to run one way. Check that the arrow on the back points the way the belt travels.
• An old or dried-out belt. Belts stored somewhere too dry, too damp or too hot can split at the joint. See storing sanding belts.
• Edge damage. A belt that has rubbed the guard or a fence tears from the edge. Track it back to the centre and replace it once the edge is frayed.

The belt slips or stalls.

A belt that stops while the motor runs is usually too slack: the tension lever isn’t fully set, or the tension spring is weak. A worn or glazed drive roller slips too. Don’t press harder to compensate; check the tension mechanism.

The belt clogs or stops cutting.

• Loading. Resin, paint and fine dust fill the spaces between the grains. Clean the belt with a crepe-rubber cleaning stick: with the sander running, press the stick lightly against the moving belt until the residue comes away. It works on most wood dust and on many paints.
• Glazing. A shiny, burnished surface means the grains are dull or covered in melted residue. A cleaning stick may help; if not, the belt is worn out.
• Too much pressure. Pressing hard heats the belt and the work and makes loading worse. Let the weight of the sander do most of the work.
• The wrong belt. An open-coat belt clogs less on softwood and paint; a coarser grit clears faster. See grits and abrasives.

Burn marks on the work.

Burn marks come from a worn or clogged belt, too much pressure, or holding the sander still. Keep it moving, clean or replace the belt, and empty the dust bag or check the extraction: a sander that can’t clear its dust runs hotter.