What it is: Chatter marks are consistent sanding imperfections across the grain of the wood varying from 1/4 inch to 1 inch apart. Wave marks are two or more "upsets" occurring along the direction of travel of a sander. They generally are 1 to 3 inches from peak to peak

Cause:

• Most chatter marks are caused by the sanding drum. It may be out of balance, out of round, have hard spots, have incorrect paper installation or have compressed rubber. Also,
  
• Poor splice/seams on the abrasive belt, drive belts and fan belts
  
• Running the big machine in the wrong direction (for belt sanders, right to left; for drum machines, left to right)
  
• Worn pulleys
  
• Bad bearings in the fan housing
  
• Loose flooring.
  
• Most wave marks are caused by imperfections already present on the floor that are transferred through the wheels of the big machine to the sanding job.
  
• Wheels on the big machine that are out of round.
  
• Improper electical hookup - voltage that is too high or low.
  
• Undulation of the floor from inadequate joist design.

Cure:

First, the problem with the big machine must be determined and fixed. Then, use a hard plate, paper disc or multi-disc sander. A sanding screen only highlights the chatter and causes the floor to dish out. Use a disc sander and hard plate while working right to left, traveling down and back in the same path and working the disc sander at slight angles for the best cut of the unit. Repeat the same procedure, overlapping the last cut one-half the size of the first disc.

Another technique is to do a light sanding with the big machine at a 7-degree angle, then go over the floor again, this time straight.

If using a multi-disc sander, walk slowly with the grain from side to side, always overlapping the unit as you move from right to left. This blends in the floor and prevents deep scratches.
If joist design or loose flooring is the problem, the structural flaws must be corrected before the floor is resanded and refinished.