Besides the rental drum sander cannot be tuned up like I am about to instruct, and will always cause "chatter" marks. These will show up as stripes across the grain of the boards once you stain, and by then it's too late. These rental machines are just too lightweight to do a decent job. And it does take months of training to use a floor sander without gouging your fine wood floor.
"Tuning up" your professional floor sanding machine is essential. Even the best of these machines can create those dreaded chatter marks. I first start by tightening the rear wheel. And I make sure all the wheels are free and spin well. Then I will take any sandpaper off the drum, and place a new sheet of 60-grit sandpaper on the floor below the drum. I stand on both edges of the sandpaper to keep the paper tight to a level floor (or have a trusting assistant do this), turn on the sander and lower the drum slowly down to the floor.
Just do this once, and you will see how level the drum is sanding. I like to have my drum sanding just a tad to one side. I can see this as the pink drum rubber is just a bit thicker on one side of the sandpaper I'm holding to the floor. But if you find that the drum is dramatically sanding to one side, adjust it so that it runs flat, or like I said, only very, very slightly to one side. When you have it running favoring one side this will always dictate the direction your floor machine will travel across the floor. You will always want the feathered edge side of the drum to be the trailing edge of where you have already sanded. For instance if the sanding drum is tilted down slightly to the left (as viewed standing behind the machine), you will always sand the floor starting on the right side of the room and work you way row by row to the left. You can see that the feathered (high) edge of the drum will sand the floor last in this case and will leave less of a drum mark. You can also make the drum flat if you wish, but it rarely stays flat. I would rather know which direction to sand instead. Oh, and be sure to keep the dust bag on the non-feathered side also or in the center. It keeps this rather top heavy machine from tilting. I would also like to suggest that the pulley will affect the drum feather at high speeds. So I choose the non-feathered side of the drum to be the on the same side as the pulleys. This has worked best for me.
And speaking of the drum rubber, if it's too old or worn down, it may not have the cushioning effect that is needed to keep the drum riding smoothly. If this is so, have the drum rebuilt and rebalanced before you attempt any more custom stain jobs.
I have for the last 5 years used anti-vibration pulley belts on my floor sander, instead of the standard ones that come with the machine. These are sold at www.leevalley.com, and are called link belts. The easy way to see these marvelous belts is to simply click on this http://www.leevalley.com/wood/page.asp?page=30051&category=1,240,41067&ccurrency=1&SID=
These pulley link belts are easily adjusted to any machine, and never seem to wear out. They will help reduce vibration even further. Use these for sure if you do any stain work, as they will absorb some more of the motor's vibration. The only disadvantage is that they make a shrill whistle as they cut the air at high speed. But the ear protection you wear (I hope you do) will make this unnoticeable. And I have found that they will never develop a "set" when left on the machine for a long time. If you insist on using those regular one-piece belts, release the tension at the end of every day, so that they don't develop a flat spot.
The last thing you need to do is to carefully apply the sandpaper to the drum or belt-sanding machine. You want the paper installed evenly and with no flat spots. On the newer belt machines there should be no flat spots on a good quality sanding belt, so just make sure that the paper travels evenly. But be sure to release the tension on the belt if you have stopped the machine to take a break. Always hang those expensive belts on a wide tube so that it won't be creased. And never lay these sanding belts on a concrete floor. They will loose their tracking ability as one side of the belt wicks moisture and expands. Oh, and speaking of storage, keep all sandpaper at 60-80 F and about 45% relative humidity if you want consistent results from your rolls or belts. Use really good quality belts, then there will be not flat spot in the belt were it is joined.
Also be sure to blow out all the dust that may have collected inside your drum or belt system after each job. A dust clogged drum will certainly cause the dreaded chatter marks. This is a good idea anyway just to remove the-grit from ruining the bearings.
The drum sander is a bit trickier, if you cinch the sandpaper too tightly it will easily create a flat and chatter prone spot on the drum. You must have the paper tight enough, and perfectly aligned just before you use the tightening wrenches. You will find that a good quality cloth backed sandpaper cinches better on these drum sanders. Cheap paper backed sandpaper can actually rip during cinching or create a bump of loose paper on the drum.
A quick flick of the wrenches is all you need so the sandpaper cinches immediately. Any further tightening will compress the rubber and create that annoying flat spot. I hope at least you have a drum that has the sandpaper slot cut on an fairly high angle to the drum. And you will have to use the proper sandpaper shims in this slot, to aid in this instant cinching of fine sandpaper I just described. I find that the better quality cloth backed sandpaper cinches better that the paper backed variety. Get this last thing wrong and you will have a chattered floor, and an angry client.
If your machine is still leaving chatter marks, take it in to your local machine shop and have them balance the drum. At the same time have the wheel bearings checked, and even the vacuum system looked at. Almost anything out of balance on this fast spinning machine can build up a vibration, which is then transferred to the floor. I am lucky to have Galaxy Electric nearby. They will simply exchange the old worn or out of balance drums for a new reconditioned one. You may find a few of these companies will handle all this through the mail also.
Step 1: How To Sand Wood Floors: Without Leaving Machine Marks Introduction
Step 2: How To Sand Wood Floors: Can it be Sanded?
Step 3: How To Sand Wood Floors: Start with Fine Tuning Your Drum Sander and Choosing the Best Sand Paper
Step 4: How To Sand Wood Floors: The Right Sand Paper Makes The Job For The Initial Rough Sanding 36 Grit
Step 5: How To Sand Wood Floors: Repairing Gaps and Second Sanding Stage With 60 Grit
Step 6: How To Sand Wood Floors: Final Drum Passes with Burnished 80 Grit or Regular 100 or 120 Grit
Step 7: How To Sand Wood Flooring: Edging - Using The Edger Without Effort
Step 8: How To Sand Wood Flooring: Screening and Final Preparation For Staining or Finishing