Love and care of your sealers and finishes does not end when you get to the job site. You have taken care selecting and housing them, and have taken them for a drive. Now, you have just carried them across the threshold of a new house (so don’t tell them that you will be putting them down and leaving in a matter of days). What now, Romeo? Keep reading.
NUTS AND BOLTS
First and foremost, know and follow the directions for the product, including, but not limited to:
- whether to stir, shake or “gently rock,”
- how long to mix,
- when to add the cross-linker or hardener (for two-part products), how to mix, and for how long,
- how long to let it rest before application,
- application tools,
- coverage rates,
- recommended application temperatures and relative humidity,
- substrate preparation,
- products to use before/after said coat,
- when to apply said/succeeding coats,
- abrasion between coats, and
- how many coats may be applied in one day.
Remember, there are reasons why those instructions are on the label – testing has proven that following them will yield the highest probability of success. Of course, circumstances frequently occur that fall outside the guidelines; do not be afraid to call your manufacturer. We may not enjoy you interrupting a scintillating sales meeting, but we really do want to help you to be successful. I should note here that manufacturers are only liable for product replacement, not your labor, meals, gas, golf rounds, pedicures, or other resand/recoat expenses. A few of the above bullet points bear further exploration.
Hot coating is not always bad, but do always defer to the manufacturer instructions for the limits: “May be recoated after X hours…,” “Allow to dry X hours…,” etc. These are set to ensure that enough drying and coalescence have taken place to hinder neither the integrity of the first coat nor the application of the next one (adhesion is usually not an issue with the successive coats with waterbornes).
When coated too early, isocyanate crosslinked finishes, more so than one-components, can be more “grabby,” as that is exactly what they are doing: grabbing the still-reactive surface of the preceding coat, as they crosslink with those components to a higher degree than if that coat was drier. One consequence is to prevent the coat from flowing out and leveling like it wants to, from those little chemical “handshakes” reaching up from below.
Most manufacturers recommend no more than two waterborne coats per day. This limit is because each coat drastically limits the rate at which solvent/water can escape and oxygen can permeate to cure the bottom coat. This decrease is closer to exponential than it is to double. Imagine, then, the decrease in permeability after a third coat. This translates to a slowing of the hardness, chemical resistance, and other properties development – a problem for floors used too quickly (because people always stay off their floors for as long as contractors tell them to).
If you are going to do more than two coats per day, stack the odds in your favor. Try to use isocyanate or aziridine crosslinked finishes for the intermediate coatings, as these are not dependent on oxygen filtering through to crosslink and cure, like one-components. Also, wait the longest time between the first and second coat – not between the second and third coat. Remember that each successive coat typically drastically reduces the solvent and oxygen permeability of the previous coat(s) – the longer you can wait after the first coat, the faster your system will cure, and achieve the properties on which you sold your customer.
DO YOU HAVE PROPER COVERAGE?
Coverage rates are an important component of the coalescence, dry and cure equation. Obviously, to match the advertised recoat times, stick to the recommended coverage rates. Again, these will vary some from region to region (as temperature and humidity also vary).
Any time that coverage is heavier than recommended, the recoat time will be extended, for both oil-based and waterborne products. Some coatings are affected more than others. For example, applying oil-based sealers too heavy can result in a dramatic extension of recoat time, by as much as an extra day or two. This is due to the nature of the driers added (metal-based agents that help to dry the coating). Typically, these sealers have a lot of surface drying agents, meaning that they are unable to dry a thicker coating quickly.
Oil-based finishes are affected to a lesser extent since they are generally equipped with surface, mid, and through driers. Predictably, one-component waterbornes are affected more than polyisocyanate-crosslinked two-components, since they are dependent on oxygen to crosslink them. This makes it easier for a heavy coat to skin over on top too early, slowing down the dry, cure and recoat time of the rest of the coating.
Be aware of low spots, as these will naturally accumulate more product/area than in flat areas, in some cases the equivalent of several coats of finish. This increased accumulation will dictate the recoat times of the entire floor, so always use low areas to determine when to apply the successive coat.
It does not take a genius to figure out that the combination of hot-coating and coating too heavy can extend the coalescence, dry, and cure of a coating system even further. A heavier coating makes it even more difficult for oxygen to get in and solvent to get out of a coating system. If every coat in a three-coat system is applied heavy, the cumulative effect may be as if a fourth coat was applied.
Finally, too thin of coverage can overwork sealers or finish, possibly resulting in bubbles, improper leveling, and other undesirable outcomes. Too heavy of a coat can also cause problems. Aside from extending dry/recoat times, air bubbles from application or shaking, especially if not allowed to “rest” before use, can by the time they work up to the surface, be prevented from popping by a surface film that has already started to flash, leaving permanent bubbles. This can be exacerbated by cooler floor and room temperatures.
THE OTHER HOT COATING
On hot summer days, most contractors know to stack the odds in their favor by coating early in the morning or late evening or night. Both are excellent ideas, especially with isocyanate-crosslinked finishes since their reaction rates can double/pot-life can shrink every 10 or so degrees above 75˚F.
In front of windows, the sun can heat up the floor and cause the finish to flash and set up prematurely. Consider hanging black plastic to block the sun. This does not have to be as involved as draping for dust and should be done the day before coating, or long enough before to ensure that the sun has not already heated up the floor areas that it hits. Often, this means only having to drape off only part of a window, also making it easier to use (and reuse).
ARE YOU READY?
Coat oil-modified stain/sealer/finish when it is ready to coat with waterborne finishes; allow me to explain. Mineral spirits/Stoddard solvents in oil-based products tend to rise up out of the coating and sit in a layer above the floor, suppressing the evaporation of more solvent. The higher the humidity, the greater the suppression/less the air can hold. Inversely, the colder the temperature, the less the air can hold. Heat can induce more solvent to evaporate since it increases the amount that a given air volume can hold.
Extrapolating, then, if a coated space is heated to the limit of application directions, but closed up, it may be days before an oil-based stain/sealer/finish is ready to be coated with waterborne finishes. This does not mean that the stain or sealer is not behaving how it should; rather, you are not supplying the proper conditions for it to happen quicker. The single best thing you can do to speed up the process is to get new air in and old air out; use fans, heat, windows, etc., to expedite the process. This is something that is better addressed with the homeowner before doing the job.
Other variables that can extend the recoat time of oil-modified stains, sealers or finishes on raw wood:
- New wood (= open/unsealed butt ends, for more absorption and slower evaporation),
- Water-popping (worse if water has not completely evaporated before staining),
- Double-staining (= more stain),
- Hot-coating over it with an oil-modified sealer or finish, and
- Situations where the air temperature may not reflect the temperature of the substrate (like in a new situation where the heat has not been on long enough, particularly in basements or new construction).
Moral of the story: When planning to coat waterbornes over oil-modified stains, sealers or finishes, know your region, including seasonal variations, and adapt accordingly. Be aware of how your substrate preparation can affect the dry time. Most importantly, use fans and windows to exchange the air in the room with new air.
Well, the final coat is down: a silky-smooth blanket of polyurethane perfection. The customers are waving and crying as you wink, turn on your heel and walk off into the sunset. And, since you treated them right, your stains, sealers, and finishes will speak highly of you long after you are gone. Boy, it feels good to be a wood flooring professional.
Ethan Erickson is a Chemist at Arboritec USA in Greenwood Village, Colorado. He can be reached at firstname.lastname@example.org.