The process of sanding a wood floor is more than just something that can be taught through a book, video, or user manual. Sanding is an art as well as a skill. Those of us who sand floors understand our equipment in a way that is difficult to explain. Whether it’s how the machine feathers, where the machine cuts, how we clock, wheel placement, the rhythm of our step, the pitch of the motor, or just the feel of the sander over different species, every machine is unique. A perfect install can be ruined by a poor sand job, just as a perfect sand job can make a poor install look good.
I think we’ve all learned from our mistakes in an effort to become better floor finishers. The following are many of the sanding-related irregularities that are more specifically detailed in the most current edition of Problems, Causes, and Cures (Publication C200) available at nwfa.org.
Sanding Marks (Chatter, Wave, Dishout/Shellout)
Sanding imperfections seen as drum marks, side cuts, sanding scratches, or swirl marks, slight unintentional indentations causing ripple-like wave effects or “bars” that run perpendicular to the wood’s grain direction on the surface of a wood floor that can become visibly pronounced in direct lighting. Indentations, or waves, usually repeating undulations, visible on a site-sanded wood floor surface, most often visible after a finish has been applied. Dishout in the surface of individual boards or within the wood floor, resulting from softer areas having been sanded to a lower level than harder adjacent areas.
These are just a few of the common causes for sanding marks
- Sanding process related:
- Poor workmanship/sloppiness
- Running the machine in the wrong direction or an inadequate first cut angle
- Running the machine too slow, too fast, or at an inconsistent pace, or an inconsistent operator walking speed, or too much drum pressure
- Not enough overlap with each pass of the big machine, the buffer, or the multi-head sander
- Power surges during operation/inadequate power to the machine
- Running the machine outside of the manufacturer’s recommendations
- “Heeling” the buffer or improper clocking of the buffer or edger
- Grit or debris left behind on the flooring that wasn’t adequately vacuumed or removed during the sanding process, affecting the subsequent scratch patterns
- Skipped steps in the sanding process
- Knots or density differentials in the flooring causing sanding inconsistencies
- Sanding of a floor assembled of mixed species of different hardness characteristics such that material is removed at different rates through the sanding process (e.g., borders, feature strips, medallions)
- Machine related:
- Improperly installed paper on a floor sander
- Poorly maintained machine
- Abrasive related:
- Poor-quality or improperly manufactured abrasive
- Contaminated or damaged abrasive
- Improper abrasive selection or improper abrasive sequence
- Site conditions:
- Substrate-related issues causing equipment malfunction
- Settled or otherwise unlevel or loose flooring
- Harmonic vibration:
- The structural integrity of, or undulations in, the subfloor
- The undulation of the floor from joist truss deflection
- Improper subfloor thickness, insufficient subfloor orientation (parallel vs. perpendicular), seams not ending on joists
- Undersized joists, undersized beams/supporting joists, improper joist or beam span (end-to-end or between joists/beams)
- Flooring installed parallel with floor joists
- Insufficient or loose fasteners, incorrect fastening schedules, etc.
- Structural integrity of the floor being sanded
- Sanding of loose, poorly installed, or floating wood floors
- Older, historic wood flooring installed directly to floor joists resulting in more deflection between the joists
- Any subfloor or wood floor system with “built-in-give,” such as with some gym floor systems
Refer to NWFA Wood Flooring Sand and Finish Guidelines for proper sanding processes.
- The structural integrity of the subfloor system is not the responsibility of a wood flooring contractor unless he or she installed the subfloor system.
- Structural-related issues should be addressed by a qualified professional.
- Resand the floor at a minimum 7- to 15-degree angle with the big machine on the first cut to flatten the floor. Sanding at the opposite angle on the subsequent cut may also be necessary to flatten the floor properly. Avoid cross-grain sanding (perpendicular to the direction of the grain) on any area of the floor if possible.
- No repairs may be necessary.
One of the most important factors to keep in mind is that every wood floor will show sanding marks on the finished surface. The reason is that every floor that gets finished requires sanding.
Some sanding marks may be visible, and some may not. Evaluating these sanding marks is where some ambiguity comes into play.
The standard for evaluation is based on viewing a wood floor within the scope of the NWFA Guidelines in order to determine acceptability:
- Evaluation of a wood floor must be done from a standing position on the floor being assessed.
- Evaluation of a wood floor must be conducted with ambient lighting. Glare from direct light sources must be avoided
- Inspection by an NWFA-Certified Wood Floor Inspector may be conducted to determine the cause, which may include a careful and critical examination of the flooring system. Inspection may take place from the floor level, with the assistance of magnification, and/or through destructive testing to determine the cause of irregularities, but not necessarily to determine acceptability.
Brett Miller is VP of Education & Certification at the National Wood Flooring Association in St. Louis. He can be reached at firstname.lastname@example.org.