Hardwood Floor Finishes for Refinishers

Why this matters

The finish is the part of a refinish the customer lives with, and it is chosen in a five-minute conversation that usually turns on one word: "shiny", "natural", "amber" or "durable". The wrong family costs either the schedule (a customer who needed the furniture back on Friday gets a finish that cures for a month) or the floor (a kitchen with a large dog gets a finish that cannot be spot-repaired). Picking well means knowing how each family looks, how it cures, how it recoats and how it fails, and reading the maker's data sheet for the numbers.

Safety comes with the can

Finish safety has three parts, and the first two are set up before the first coat is opened.

Flammable vapor. Oil-modified and moisture-cure urethanes, many penetrating oils and solvent-borne stains give off flammable vapor. Before opening them, shut off every ignition source in the work area and any space sharing its air: pilot lights and gas appliances off, open-flame and spark sources off, and the HVAC system handled the way the finish maker directs so vapor is not pulled through the house. When vapors have cleared, relight each appliance per its own instructions and confirm the burner lights and runs before leaving. If anyone smells gas at any point, everyone leaves the house without touching switches and the call is made from outside.

Breathing it. Solvent vapor calls for an organic vapor cartridge respirator, and two-component waterborne and moisture-cure finishes can carry isocyanates, which are respiratory sensitizers: an exposure that causes no symptoms today can cause asthma later. Respirators are used only under a written program with medical evaluation, fit testing and a cartridge change schedule, per 29 CFR 1910.134 (applied to construction work by 29 CFR 1926.103). Read the SDS for the specific product; it names the route and the respirator type. A crew member not in the program does not apply those products. Nitrile gloves and eye protection cover the skin and splash routes.

Oily rags. Rags, pads and applicators wet with drying oils (oil-modified urethane, penetrating oils, hardwax oils, oil stains) give off heat as the oil cures. Bunched in a bag or a pile, the heat has nowhere to go and the rag can ignite by itself hours later. Lay them flat to dry outdoors away from buildings, or put them in a closed metal container filled with water, then dispose of them as your local rules require. They never ride or sit in the truck cab, and they never go in the customer's trash can. Waterborne finish rags do not cure by oxidation and do not carry this hazard, but the solvent-borne stain used under them may.

The finish families

Acid-cure (conversion varnish) finishes also exist on refinish work, mostly with specialists; they release formaldehyde during cure and have their own handling rules. The cure chemistry behind every row, and why humidity speeds one and slows another, is in Humidity and Cure: Moisture-Cure, Evaporative and Reactive.

Two rules hold across the film-forming families:

• Stay in one maker's system. Sealer, stain and topcoats are tested together. An oil stain under a waterborne topcoat is allowed only after the stain maker's dry time, or over the sealer the topcoat maker names; skip that and the topcoat can fail to bond.
• Recoat inside the window and abrade when told. Each data sheet gives a recoat window and says whether to abrade between coats. Miss the window and the next coat sits on a cured surface without a mechanical key, which is how a recoat peels in sheets (worked in The Recoat That Peeled Off in Sheets).

Sheen is a separate choice from family. Gloss, semi-gloss, satin and matte versions of the same finish differ by the flattening agent added, which scatters light. Lower sheens hide fine scratches, dust and the slight unevenness of an old floor; gloss shows every one. The flattening agent settles in the can, so stir it fully from the bottom (shaking a waterborne finish puts bubbles in the film), and where a room takes more than one can, box them: pour the cans together into one clean container so the whole floor gets the same sheen. Only the last coat sets the sheen on most systems, and some makers call for gloss in the build coats with the chosen sheen on top; the data sheet says which.

Air rules. VOC content limits for floor finishes are set federally by EPA's architectural coatings rule (40 CFR 59, Subpart D) and more tightly by some states and regional air districts. A product sold legally in one state can be prohibited in the next; buy from the line your state allows, and do not carry high-VOC stock across state lines to a job.

Worked example: two finishes against one deadline

A 600 sq ft living and dining room in red oak, sanded through day 1. The family needs furniture back by the end of day 5 and has a large dog. The owner is choosing between a two-component waterborne system and an oil-modified urethane.

Say the data sheets read, at 70 F and 50 percent relative humidity (illustrative values; read your own):

• Waterborne 2K: a sealer coat plus two topcoats; recoat after 3 hours; light foot traffic 24 hours after the last coat; furniture after 72 hours; rugs after 7 days.
• Oil-modified: three coats; recoat after 12 hours, which on a day crew means one coat per day; light foot traffic 24 hours after the last coat; furniture after 4 days; rugs after 30 days.

Waterborne 2K schedule. Day 2: sealer at 8 am, first topcoat at 11 am, second topcoat at 2 pm (8 plus 3 is 11, 11 plus 3 is 14). Light traffic from 2 pm on day 3. Furniture 72 hours after 2 pm on day 2, which is 2 pm on day 5. Rugs at 2 pm on day 9.

Oil-modified schedule. Coat 1 day 2, coat 2 day 3, coat 3 at 8 am day 4. Light traffic 8 am day 5. Furniture 4 days after the last coat: 8 am day 8. Rugs on day 34.

Against the deadline. Furniture by the end of day 5: the waterborne 2K makes it at 2 pm on day 5; the oil-modified misses by more than two days (8 am on day 8). The dog's nails favor the 2K film too. The call is the 2K, with a written note to the customer: furniture on felt pads, no rugs until day 9, damp-mop only with the maker's cleaner.

The condition that flips the schedule. Those times are at 70 F and 50 percent RH. If the house is closed up at 60 F and 75 percent on day 2, a waterborne finish dries slower and the 3-hour recoat can stretch; recoating a film still soft underneath traps water and clouds or wrinkles. Read a thermometer and hygrometer at floor level before each coat, run the HVAC to bring the house into the maker's application range, and touch-test an inconspicuous spot rather than reading the clock. A moisture-cure finish would move the other way in that humid house, faster and with a shorter working time.

When a different family is the right answer

• The customer wants a deep amber. Oil-modified urethane gives it naturally; an ambering waterborne or an oil-based sealer under waterborne gets close on a shorter schedule. Show a sample board finished in the actual system on the actual species.
• A kitchen that will be scratched in one spot. Penetrating or hardwax oil lets a scratched area be cleaned and re-oiled without screening the room. The trade is regular maintenance on the maker's schedule, which the customer has to want.
• An unknown existing finish on a recoat job. Test adhesion on a small area before committing; wax, polish residue or an incompatible old finish decides the job before the family does. See related: How to Screen and Recoat a Worn Floor Finish.

How to verify the finish went down right

• Before each coat: temperature and humidity at floor level inside the maker's range, written on the job sheet with the time.
• Recoat timing taken from the data sheet and confirmed by touch, never by the clock alone, and abrasion between coats done where the system requires it.
• After the last coat, judged standing in normal light: even sheen, no lap marks, no trapped debris in the traffic path. Raking a work light across the floor will show flaws a customer standing at the doorway will never see.
• Rags and applicators handled as above before the crew leaves, and every appliance relit and confirmed running.

References

• Finish manufacturers' technical data sheets and SDS (dry, recoat and cure times; application conditions; respirator type)
• OSHA 29 CFR 1910.134, Respiratory Protection, applied to construction by 29 CFR 1926.103
• EPA 40 CFR 59 Subpart D, VOC emission standards for architectural coatings; state and regional air district rules
• See related: Humidity and Cure: Moisture-Cure, Evaporative and Reactive; How to Sand and Refinish a Hardwood Floor; The Recoat That Peeled Off in Sheets