Sanding Previously Finished Wood Floors

NWFA

Brett Miller, Vice President of Education & Certification at the NWFA
Brett Miller, Vice President of Education & Certification at the NWFA

By Brett Miller, NWFA Vice President Technical Standards, Training & Certification

One of the many benefits of wood flooring is that it can be refinished numerous times throughout its service life. This bodes well for flooring contractors who are trained to handle the many complexities involved in sanding, finishing, and refinishing wood floors because their skills are in high demand. In fact, many experts say that there are enough wood floors already in existence today to keep wood flooring sand and finishers busy for many decades to come.

When refinishing previously finished wood floors, there are many details that must be considered, including:

• The condition of the existing finish and floor
• The possibility of lead
• If the floor was site-finished
• Previous finish color
• Wear layer thickness
• If the floor was previously factory-finished
• If the floor is beveled
• The condition of the substrate and flooring system
• The species
• The width
• Pre-existing conditions, such as the age of the floor and moisture problems

Each of these considerations may affect the process you use to achieve the best possible refinishing result.

It is not necessary to fully sand the floor to restore the finish unless the floor has visible dents, wear patterns, or permanent cupping, or if the customer wants to change the color of the floor. Abrasion and recoating may suffice.

Before sanding a previously finished wood floor, determine whether the floor finish contains lead. Any floor finishes applied before 1978 may contain lead. In the United States, the EPA requires lead-safe certification and proper abatement procedures when lead is present. Test kits are available to determine the presence of lead in floor finishes and other architectural coatings. Make sure to abide by local, state, and federal guidelines for testing, handling, and disposal of lead-based products. Failure to do so can result in significant fines and serious health concerns. For more information, visit the U.S. Environmental Protection Agency website at www.epa.gov/lead.

If the floor was previously site-finished, use the least-coarse abrasive necessary to remove the previous finish. When old paint, shellac, or wax is present on an existing floor, move the machine quickly using a light pressure setting to minimize heat. Allowing these types of finishes to heat up will liquefy them and may damage your equipment.

When re-sanding a dark-stained or white-stained floor and changing the color, the floor can exhibit residual stain in the soft grain, voids, and cracks, which may require a more-aggressive first cut in order to remove the previous color. In some cases, it may not be possible to completely remove these types of stain.

The number of times a given floor can be sanded depends on the skill of the person sanding the floor, the type of equipment used, the thickness of the remaining wear-layer, and the flatness of the floor. Generally, if the wear thickness is less than 3/32″, the floor should not be sanded. Evaluate wear-layer thickness in several areas and check for flatness to determine if you should attempt sanding. Measurements often can be made at floor registers or by removing transition moldings. Where there are gaps between boards, a feeler gauge may be used to measure the thickness of the flooring down to the top of the tongue. Use caution with this method as it works well with solid wood flooring but may not be accurate with some engineered flooring. Also, the wear layer on some engineered wood flooring may not be as deep as the tongue and may not be able to be refinished.

If the floor was factory-finished, determine the type of finish on the floor. High-abrasion finishes such as aluminum oxide may be difficult to sand. These finishes may respond better by using a fine-grit (80-grit or finer grit ceramic-fired) abrasive as the first grit used to remove these surface finishes, followed by a coarser grit to begin the upward grit progression to flatten the floor and refine scratches.

When sanding eased- or beveled-edge flooring, the appearance of bevels may not be consistent after sanding. In the case of a micro-bevel product, it is possible the bevel will be eliminated. The sanding process may require extra cuts in order to eliminate these eased edges. When taking these additional steps, be sure the floor is capable of being sanded. When maintaining the beveled edges, carefully clean and/or scrape the bevels, being careful not to damage the face of the board. In addition, make the customer aware that sanding a beveled-edge product will change the profile of the bevel and the look of the floor.

Further, loose substrates, movement, or vertical deflection in the flooring system may cause wavy or chattered appearances in the final sanded product and are often unavoidable. Here are a couple of suggestions that may help minimize them:

• Sand the floor at a 7- to 15-degree angle all the way through the final sanding phases. This also will require additional sanding with a hard plate, multi-disc or planetary, or oscillating machine to properly remove scratch marks in the flooring surface and remove the condition.
• When possible, hire a professional to secure the substrate from below per local building codes.

Some species are more difficult to sand than others. Some species are also photosensitive and may have changed color over time. Re-sanding these floors will result in a change in the floor’s color.

In general, wider width flooring will require a steeper angle on the first cut to achieve a flat surface. When wider plank flooring is cupped or crowned, be sure that the moisture content has stabilized and is adequate prior to sanding.

Pre-existing conditions such as moisture issues and age may also affect the sanding process. Cupped floors should not be sanded until the moisture problem has been corrected and the moisture content of the wood flooring and the subfloor have stabilized and are equalized.

Finally, older installations that undergo the refinishing process may experience filler-pop. This is the result of movement of the wood flooring.

The National Wood Flooring Association has detailed information about sanding and finishing wood floors available through NWFA University, an online training platform that is convenient and affordable. Visit nwfa.org/nwfa-university.aspx for more information.

The left drum is new and how they should look. The second one in the middle looks like the contractor hit a nail or staple, and a cord. Also there is a buildup of wood particles that needs to be cleaned off. At right, is a drum after dressing was applied, which is not recommended by the manufacturer of this drum.
The left drum is new and how they should look. The second one in the middle looks like the contractor hit a nail or staple, and a cord. Also there is a buildup of wood particles that needs to be cleaned off. At right, is a drum after dressing was applied, which is not recommended by the manufacturer of this drum.
Photo 3: A sanding machine side cover damaged by improper belt alignment.
Photo 3: A sanding machine side cover damaged by improper belt alignment.
Photo 7: Apply adhesive to the subfloor in a thickness that will match the overall elevation of the floor.
Photo 7: Apply adhesive to the subfloor in a thickness that will match the overall elevation of the floor.
Photo 6: Delaying medallion installation may allow the field to expand beyond the outline.
Photo 6: Delaying medallion installation may allow the field to expand beyond the outline.
Photo 5: If you expose nails on any pass, use a nail set to drive the nail further into the floor and out of the way of the router bit.
Photo 5: If you expose nails on any pass, use a nail set to drive the nail further into the floor and out of the way of the router bit.
Photo 4: Make at least three to four passes at increasing depths to avoid overheating and damaging the router bit.
Photo 4: Make at least three to four passes at increasing depths to avoid overheating and damaging the router bit.
Photo 3: Using a plunge router and bit, cut away the existing flooring on the inside of the template.
Photo 3: Using a plunge router and bit, cut away the existing flooring on the inside of the template.
Photo 2: Fastening medallion template to main field of flooring
Photo 2: Fastening medallion template to main field of flooring
Using a magnet to find fasteners
Using a magnet to find fasteners

About the Author:
Brett Miller is the Vice President of Technical Standards, Training & Certification at the National Wood Flooring Association where he is responsible for the NWFA’s technical training programs, as well as the NWFA’s technical guidelines and standards.

About The Author

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