How Rippling Happens
Rippling, or chatter, happens when you sand a floor with a drum sander. As you move the sander along the floor, the action of the drum can make the machine bounce up and down, and every time it lands, the drum makes a slight depression in the wood. You often don't notice that you're making chatter marks while you're sanding but they become apparent when you finish the floor, as the play of light on the glossy surface highlights the depressions. By then it's usually too late to do anything about it short of starting the job over.
Condition of the Sander
Most rippling is the result of problems with the drum sander, which refinishers often call the big machine. If the housing on the big machine isn't heavy enough -- which is common with rental machines -- the machine has a greater tendency to bounce. Even if the housing is heavy enough, the machine could have defects such as a misaligned or misshapen drum or ineffective paper grips. The problem can also be electrical. If the voltage to the machine fluctuates or is insufficient, the motor may labor and run inconsistently.
If you are sanding an uneven floor with a light machine, it's almost impossible to avoid chatter, but you can minimize it by cutting across the grain at a 7- to 15-degree angle on the first pass. If that doesn't work, it may be time to upgrade to a heavier, more sophisticated machine, at least for this project. Top-of-the-line drum sanders have a clutch that allows you to lower the drum by lowering a handle so that you don't have to raise and lower the entire housing. Their weight can be 100 or more pounds greater than basic machines, and they bounce far less.
It's always a good idea to check for ripples before you finish the floor by wiping the floor with a damp cloth and then looking across the surface as the light shines from in front of you. Forget about trying to fix any ripples you find with the same drum sander that created them -- you'll probably just make them worse. A better strategy is to re-sand the floor with an orbital floor sander or a flooring disk sander. The latter is not the same thing as an edger, which you use to sand the edges of the floor. You should be able to find either machine at a rental outlet.