At the home center, they sit on the shelf as neighbors and they even share a last name, but oil-based and water-based polyurethanes are different kinds of finishes for different needs. Whether finishing floors, trims, doors, or any wood product, once you learn the difference between the two, the choice will be easier.
What Polyurethane Is and Why It's Beneficial
Polyurethane is a clear thin liquid coating that can either be brushed or sprayed on. The purpose of the coating is mainly protective. But polyurethane does bring out the grain in wood finishes and impart a golden tone.
Polyurethane finishes are often compared to varnishes, shellacs, and lacquers. Yet the unifying factor of polyurethanes is that they are essentially plastic. Shellac, notably, is made from an organic product—insects—yet polyurethane is a polymer coating produced in the factory.
Polyurethane finishes work well on interior applications. They can build a hard yet mostly clear shell while allowing the lower surface to show through.
Oil-Based Polyurethane Finishes
Oil-based polyurethane uses various petroleum and mineral solvents as the vehicle for the polyurethane solids. Unlike oil-based paints which are less widely found than before, oil-based polyurethane coatings are still widely sold and used.
Oil-based polyurethane, available both in spray and brush on formats, creates a hard protective shell in fewer coatings than with the water-based polyurethane. It does leave a slightly yellow sheen, especially with multiple coatings, though this may be desired in some applications.
Oil-based polyurethane dries slowly. One coat usually dries to the touch in about two hours, with additional coats allowable in about six hours, depending on room conditions. During this time, the odor of the oil-based polyurethane is sharp and pronounced, and many people find it objectionable. By the time the product has fully cured, the odor will have gone away. Oil-based polyurethane requires mineral spirits or paint thinner for cleanup.
Oil-based polyurethane is self-leveling, which means that it will become smooth when applied to a horizontal surface.
When to Use Oil-Based Polyurethane
Oil-based polyurethane coatings provide excellent abrasion and scratch resistance. This makes them a good choice for wood floors or for any application such as cabinets, railings, or countertops where durability is critical. Oil-based polyurethane coatings achieve a rich, golden glow in few coats.
You’ll need to allow for extra curing time if you want to apply multiple coats. But oil-based polyurethane does have such a high build and is so strong that you often can get by with just one coat for some lower impact applications.
You’ll want to confine your usage of oil-based polyurethane finishes to the interior only when plenty of ventilation is possible or when the home will not be occupied during the curing. Oil-based polyurethane requires patience and time for the best results.
Oil-Based Polyurethane Pros and Cons
Fewer coats required
Slight brown, golden, or yellow coating, if desired
Long curing time
Sharp odor while drying
Requires mineral spirits to clean up
Sticky until fully cured
Water-Based Polyurethane Finishes
Water-based polyurethane finishes use water rather than solvents as the base for carrying the polyurethane solids. Like the oil-based polyurethane, water-based polyurethane can form a hard protective film, but more applications are required.
Because water-based polyurethane dry quickly, more coats can be applied in less time than with oil-based products. Water-based polyurethane begins milky-white in the can but dries clear. Very little, if any, color is imparted by the water-based polyurethane.
Water-based polyurethane coatings have almost no odor, and only soap and warm water are required to clean up this variety.
When to Use Water-Based Polyurethane
You’ll want to use water-based polyurethane if you have any aversion to dealing with messy, odorous solvents. Water-based polyurethane finishes are easy to apply and easy to clean up after.
If you only need a thinner, more flexible coating, water-based polyurethanes might be right for you. Even one coat of oil-based polyurethane is thick, but with the water-based product, just one or two coats applies nearly invisible and with almost no build.
By the same token, if you do want a hard shell look with the water-based product, numerous coats are required to produce that effect.
Water-Based Polyurethane Pros and Cons
Easy clean up with water
Flexible finish when applied with fewer coats
More coats required for a hard shell
Less durable against scratches
Difficult to see where product has been previously applied
Ventilation Guidance for Spray Polyurethane Foam Application. U.S. Environmental Protection Agency