From a finish manufacturer’s perspective, there has always been a preponderance of technical support calls (read: issues!) with coating water-based finishes over oil-based stains, sealers, or finishes. Along with moisture issues with wood/subfloor/concrete, these two topics probably account for the lion’s share of contractors’ issues.
Find out the condition of your job site
As manufacturers, we formulate our finishes and stains to work optimally in a specific temperature and humidity range, which is usually clearly stated (often, more than once) on the back label of the product. The closer that you get the job site conditions to these recommended conditions, the better your results will be. If this means turning up the heat or turning on the AC, do it. For new construction, wait until HVAC is up and running a minimum of five days before installing, sanding, and coating. Or if this is not possible, ask the builder to provide temporary HVAC that replicates those conditions.
Obviously, the first step for new or existing flooring, is to assess the job site and, accordingly, educate the builder/homeowner/check-writer as to the timetable for the job, and why. Building in extra time for the marriage of “Family Oil” with “Family Water” will, in the end, make married life much easier.
Namely, the higher the humidity, the less room the air has to hold mineral spirits = the lower the evaporation rate of mineral spirits from a stain. Hence, low temperatures, high humidity, and no new/old air exchange = the longest-possible time for the stain to be ready to coat with waterborne sealers or finishes.
However, how often do actual job site conditions mirror the recommended conditions? A contractor in the summer in Florida (high temperatures and high humidity) faces vastly different conditions than one in winter in Colorado (low temperatures and very low relative humidity). As such, you, the contractor, need to be an expert in your geographic area. Namely, you have had experience installing, sanding, and finishing floors in all seasons in your region – pay attention so that you’ll know how to adjust your finishing schedule seasonally.
More so with newly installed floors or lace-ins. Why? If you recall, wood is, in a sense, comprised of miniature “straws” that run horizontally (in plain-sawn wood), opening at the butt joints on either end. These straws once transported sap and water up (in the spring and summer) and down (in the fall and winter), during the life of the tree. Thus, when newly installed, these straws are empty (coming from the kiln at 6-9 percent, ideally), and will readily suck up, through capillary action, the initial stain or sealer applied to them. This initial uptake of proportionally more solvents means that it will also take the solvents longer to evaporate from these joints, translating to longer dry times than with a recoat or resand, where these butt ends are already filled with the old finish system. If you have ever noticed white lines on butt joints, but not along seams, on a newly installed or lace-in solid wood flooring, after applying waterborne finishes, you have likely coated the stain too early.
By extension, end grain installations will both absorb much more oil-based stain/sealer and require longer to release the associated solvents before coating with waterborne sealers or finishes.
Don’t forget about wood moisture
Wood moisture is like the “humidity of the wood.” Similar to how humidity depresses mineral spirits’ evaporation/stain dry, so too does increasing moisture levels in wood. We all know the rules for a new install, regarding acceptable moisture levels in the wood, subfloor, etc., but what about a resand in humid Wisconsin in the summertime? The wood, while well-acclimated, is going to have a higher moisture level, so you’ll have to consider the effect on the timing of your water-based top coat.
This includes water popping the stain. Always use a moisture meter, before water popping and after – only stain once the moisture level has returned to the initial value. Be sure to check any low areas, butts/seams, and enough areas for a representative background level; write these readings and locations down on a notepad, to accurately compare to the final values.
If you do not wait until the moisture levels have returned to pre-water-pop levels, you can dramatically inhibit the dry of the stain/the time before the stain is ready to coat.
Consider, too, that water popping, especially when finishing at lower grit levels and with more-porous wood species, allows significantly more stain to be applied. In addition to the extra time needed to allow the water to dry, this can really increase the time needed for the stain to be ready to be coated with waterborne sealers or finishes.
Woods vary widely. Some may have oils that impede the drying of oil-based stains, sealers, or finishes. Some take longer to dry after water popping. Keep track of wait times, etc., on different species, so you can better plan and prepare for the next one.
Other steps that you can take to get your stained floors ready to coat:
Read the label(s)
Read the oil-based stain, sealer, or finish back label. It just might be that the label indicates that the stain is ready to coat in X hours with oil-based coatings; don’t assume that it’s okay to coat with water-based coatings in that amount of time unless explicitly stated. If in doubt, call the manufacturer; we want to help you avoid problems – your success is our success, too!
Read the waterborne sealer/finish back label, too – the manufacturers may have specific recommendations for coating over oil-based stains and sealers.
Allow proper dry time
We all know that (wet) oil and water don’t mix. The key is not to overcoat oil-modified urethanes with waterborne sealers or finishes until enough of the solvent has evaporated to allow a waterborne coating to properly coalesce, adhere, and cure atop it.
Usually (but not always), coating too early is evident by a whitish or opaque haze between the stain and the water-based finish system. While this haze will usually go away with time, it is possible that the adhesion of the water-based coating system has been compromised enough that the homeowner might complain of peeling later on, from the field, seams, or both. Further, it can prevent ideal coalescence, adhesion, and general development of the film properties needed to achieve the full durability of the coating system – also resulting in callbacks.
Maintain proper site conditions
Mineral spirits, the solvent in oil-based stains, sealers, and finishes, is really a range of different components that evaporate at different rates. This range comes from the nature of how mineral spirits are produced. Mineral spirits, also known as Stoddard solvents, or generically as petroleum distillates, are just that: distillates of petroleum. When a barrel of oil is processed, it is first distilled. A refinery distillation column has numerous levels, called stages, throughout its height. As the oil is heated, the components vaporize, and then condense (distill) on these stages, according to their boiling points/molecular size, from which they are then removed. The heaviest components (biggest molecules) remain at the bottom, and the lightest components condense back into liquid and are removed at the top stages. Octane exits around the top, just above the range of hydrocarbons that typically make up mineral spirits.
Mineral spirits are composed of a range of petroleum distillates, with varying evaporation rates; the longest-chain components are the slowest to evaporate and influence the “ready-to-coat-with-waterbornes” time the most.
Now, consider that mineral spirits, due to their vapor pressure/proportionally higher molecular weights, concentrate low above the floor as they evaporate. In effect, in a closed room, as they evaporate, they will sit, like a blanket, low above the floor, preventing even more mineral spirits from evaporating.
Higher temperatures mean that the air will hold more evaporated mineral spirits, which will allow this blanket to rise higher/expand upward. Colder air depresses the amount that can evaporate, creating a denser, thinner blanket closer to the floor. What happens if the door to the room is opened? Now, you have allowed the blanket to spread out; more of the mineral spirits can evaporate.
What if you open some windows and add fans? Okay, you are getting really smart now – you have facilitated air replacement: you have allowed the saturated air to leave, continually replaced by new (un-mineral-spirits-saturated) air. Combined with added heat, this is the fastest way to get an oil-based stained floor ready to coat with water-based finishes.
Remember, as manufacturers, we formulate our products for specific, “average” conditions, both stains and finishes; read the back labels. However, as contractors, seldom do you have the luxury of experiencing these optimal conditions. So, pay attention to your seasonal challenges, and be aware of the effects of temperature, humidity, wood moisture level, and air exchange when coating water-based finishes over oil-based stains. Consider the effects of water popping, wood species, and new/lace-in wood. Educate your builders/homeowners as to your timeline, so you are not forced to compromise the integrity of your finish system, and thereby deliver a finish system that may not live up to the expectations on which you sold them. Finally, as always, call your manufacturer if you need help.
Incorporate these ideas and start coating oil-based stained wood floors with water-based finishes when they are ready to coat.
Ethan Erickson is a Chemist at Arboritec USA in Greenwood Village, Colorado. He can be reached at firstname.lastname@example.org.