By Brett Miller, NWFA Vice President Technical Standards, Training & Certification
A major advantage of real wood floors is that they can be refinished when they become damaged or start to look worn. Yet even a perfect installation job will fail with a bad sanding process. Sanding wood floors is an art, and the skill required to do it right—and do it well—is acquired through training, trial and error, and experience.
Sanding marks are one of the most common sanding imperfections. They can appear as drum marks, side cuts, scratches or swirl marks.
Human error is usually the cause of these imperfections, such as poor workmanship; running the machine in the wrong direction; running the machine at the wrong speed or at an inconsistent pace; inconsistent operator walking speed; inadequate first-cut angle; not enough overlap with each pass; too much drum pressure; power surges during operation or inadequate power.
Other culprits can include running the machine outside of manufacturer’s recommendations; “heeling” the buffer; improper clocking of the buffer or edger; grit or debris left behind on the flooring that wasn’t adequately vacuumed or otherwise removed during the sanding process; or skipping steps in the sanding process altogether.
Common machine-related issues can include improperly installed paper on a floor sander; out-of-balance or out-of-round drum or upper roller; improperly aligned upper roller assembly; debris on the upper roller assembly; wheels on the big machine that are out-of-round or dirty; bad, worn out or loose drive belts/pulleys or fan belts/pulleys; bad bearings in the fan housing or drum shaft; poorly maintained machine, bent edger pad or buffer drive plate; or improper voltage (too high or low).
Common abrasive-related issues can include poor splice/seam on the abrasive belt; poor quality or improperly manufactured abrasive; contaminated or damaged abrasive; or improper abrasive selection or sequence.
Common jobsite condition issues can include substrate-related problems causing equipment malfunction; or settled or otherwise unlevel flooring. If the cause of the issue is with the equipment, abrasives, or jobsite, it must be corrected prior to attempting repairs. Isolated sanding marks can be addressed by repairing/resanding individual areas. Resanding the affected areas may require the first cut with the big machine to be on a steep angle, followed up with straight cuts. Use of a hard plate, multi-disc or planetary sander also will help alleviate some sanding marks.
Chatter is another common sanding imperfection. It appears as a ripple-like wave effect that runs perpendicular to the grain, every few millimeters to an inch, from peak to peak. Machine-related causes of chatter include improperly installed paper on a drum sander; out-of-balance or out-of-round drum or upper roller; loose drum nut; wheels on the big machine that are out-of-round or dirty; bad, worn out, or loose drive belts/pulleys or fan belts/pulleys; bad bearings in the fan housing or drum shaft; poorly maintained machine; dust build-up within the machine; or improper voltage (too high or low).
Abrasive-related causes of chatter include poor splice/seam on the abrasive belt; poor quality or improperly manufactured abrasive; or contaminated or damaged abrasive. The machine-related and abrasive-related causes of chatter are almost identical to the causes of sanding marks.
Chatter also can be caused by harmonic vibration, inadequate structural integrity or undulations in the subfloor; joist truss deflection; improper subfloor thickness, insufficient subfloor orientation (parallel vs. perpendicular), or seams not ending on joists; undersized joists, undersized beams/supporting joists, or improper joist or beam span (end-to-end or between joists/beams).
Other causes include flooring installed parallel with floor joists; insufficient or loose fasteners, incorrect fastening schedules, etc.; inadequate structural integrity of the floor being sanded; sanding a loose, poorly installed or floating wood floor; older, historic wood flooring installed directly to floor joists resulting in more deflection between the joists; or any subfloor or wood floor system with “built-in-give,” such as with some gym floor systems.
If the cause of the chatter is related to the equipment, abrasives, or harmonic vibration, these issues must be corrected before attempting repairs. To remove chatter, the first cut should be on a steep angle, followed by straight cuts, followed by use of a hard plate, multi-disc or planetary sander.
Wave is similar to chatter, and appears as repeating indentations on the wood ranging from 1” to 3” or more, peak to peak.
Causes of wave can include the structural integrity of the floor being sanded or subfloor beneath it; foreign objects stuck on sander wheels/wheels out-of-round; knots or density differentials in the flooring, causing sanding inconsistencies; poorly maintained machine; power surges during operation/inadequate power; improper big machine sanding techniques; floor not properly sanded flat during initial cuts; inconsistent operator walking speed; improper abrasive selection/sequence; inadequate first-cut angle; operating the big machine in the wrong direction; not enough overlap with each pass of the big machine; or dust build-up within the machine.
If the cause of wave is related to structural or power issues, these must be corrected before attempting repairs. To remove wave, the first cut should be on a steep angle, followed by straight cuts, followed by use of a hard plate, multi-disc or planetary sander.
Dishout appears as undulations in the surface of the floor resulting from softer areas having been worn, hollowed or sanded to a lower level than harder adjacent areas.
Sanding-related causes can include sanding a mixed species floor with various hardness ratings; sanding perpendicular to the installed direction; an incomplete or inconsistent sanding procedure accompanied by an inadequate abrasive grit selection; using drum pressure that is too heavy for the species being sanded; using thick, compressive or doubled-up driver pads on the buffer drive plate with the edger during the final sanding processes; allowing for inconsistent material removal based on material hardness characteristics (springwood, softer species); improper buffer sanding technique; or removing springwood at a quicker rate than the summerwood.
Maintenance-related issues can include using standing water or other liquids to clean the floor; using steam mops to clean the floor; excessive or aggressive maintenance practices; heavy foot traffic; pet nails; failure to remove grit from the floor surface; or worn, dirty or inadequate floor protection under furniture.
To repair these kinds of issues, resand the floor at a minimum 7-15° angle with the big machine on the first cut to flatten the floor. Sanding at the opposite angle on the subsequent cut also may be necessary to properly flatten the floor. Avoid cross-grain sanding (perpendicular to the direction of the grain) on any area of the floor if possible.
On multi-directional floors, sand at a 45° angle to the direction of the grain where possible in order to flatten the floor. Use a hard plate, multi-disc or planetary sander to alleviate sanding marks and minimize dishout.
The National Wood Flooring Association has detailed information about sanding and finishing wood floors available through our online training platform NWFA University. For more information, visit nwfa.org/nwfa-university.aspx.