It’s important to let each coat of polyurethane dry completely before applying the next one to ensure proper adhesion and a smooth, even finish. Improper drying can result in a soft, gummy finish that may scratch or dent easily. Additionally, if not given enough time to dry, polyurethane may release volatile organic compounds (VOCs) into the air, which can be harmful to the environment and to human health when applying the next coat.
How Long between Coats of Water-based Polyurethane
Typically, you should wait at least 2 hours between coats of water-based polyurethane. Waiting 4 hours will bring the best result, and you should not avoid applying more than two coats in 24 hours.
However, you should follow the manufacturer’s instructions for the specific product you’re using, as the drying time can vary depending on the type of polyurethane and the conditions in which it’s applied (temperature, humidity, etc.).
Generally, when applying water-based poly, 60 to 90 degrees Fahrenheit is the ideal temperature range, with a relative humidity of no more than 60%.
How Long between Coats of Oil-based Polyurethane
The drying time between coats of oil-based polyurethane can vary, but it is usually around 6 to 10 hours, and should not be applied more than two coats in 24 hours. However, it’s recommended to wait at least 8 hours before applying the next coat and to follow the manufacturer’s instructions for the specific product you’re using.
For best results, make sure the previous coat has fully cured and that the surface is clean and free of any dust or debris and sanded before applying the next coat.
Thicker coats, lower temperatures, higher humidity, or a lack of air movement determines the drying time. An average air temperature of 77° (75° to 80°) Fahrenheit, and relative humidity of 50% is ideal for oil-based polys.
Time to Fully Cure Polyurethane
Do not confuse drying-time with curing-time, they are different! Curing is different that drying, and it’s the second phase between those two. We have explained drying-time in details as above; now it’s time to answer how much time does it take to fully cure the polyurethane.
Generally said, it will take between 25 and 30 days for your finish to cure, regardless of the manufacturer, type of polyurethane used or number of coats applied. But if you need a more specific answer, it requires 20 days for water-based polys and 30 days for oil-based polys to fully cure. After this point, no more chemical alterations will take place and the polyurethane covering will become inert.
Drying vs Curing
As said, drying and curing are two related, but distinct, processes that occur after the application of polyurethane or other coatings.
Drying: Drying refers to the process of removing the solvents (especially, the volatile organic compounds) from the coating, allowing it to become touch-dry. This typically takes several hours, depending on the conditions and the thickness of the coating.
Curing: Curing refers to the chemical reaction that occurs as the coating hardens and reaches its final hardness and durability. This process can take several days or even several weeks, depending on the conditions.
It’s important to allow both drying and curing to occur fully before exposing the coated surface to wear and tear or applying additional coats. Failing to do so can result in an inferior finish and decreased durability.
Applying Next Coat before Drying and Curing the Previous Coat
Applying a next coat of polyurethane before the previous coat dries and cures can result in a number of problems, including:
- Adhesion issues: The new layer of polyurethane may not adhere properly to the previous one, causing a rough and uneven surface.
- Blushing: The new layer can cause the previous layer to turn white, giving it a hazy appearance.
- Dulling of the finish: The new layer can dull the previous one, leaving an unattractive and unprofessional look.
- Delaying the curing process: The previous coat needs time to dry and cure properly. Interrupting this process can negatively impact the final outcome.
It’s best to follow the manufacturer’s instructions and allow each coat to dry and cure completely before applying the next one.
Fast-Drying vs Slow-Drying
Fast-drying and slow-drying polyurethane refer to the speed at which the polyurethane dries after application. Both have their own advantages and disadvantages:
Fast-drying polyurethane: They dry quickly, typically within an hour or two, and can be recoated within a few hours. This type of polyurethane is ideal for projects where a quick turnaround time is required.
Slow-drying polyurethane: They take longer to dry, typically taking several hours or even a full day to dry completely. This type of polyurethane is ideal for projects where a more even and thorough curing process is desired, as it allows more time for the polyurethane to level and harden properly.
Polyurethane Not Drying : Reason and Solution
Polyurethane not drying can be caused by several factors, including:
- Improper application: Not stirring the can well, over-applying or not allowing enough time between coats can prevent polyurethane from drying properly.
- Humidity: High humidity levels can slow down the drying time of polyurethane.
- Temperature: Low temperature or exposure to cold air can cause polyurethane to take longer to dry.
- Contamination: Dust, dirt, or other contaminants on the surface can interfere with the drying process.
- Poor ventilation: Poor air circulation can also slow down the drying time of polyurethane.
- Stopping cross-linking: The oils in the wood you’ve applied the coating to are stopping the cross-linking process.
To resolve the issue, it is recommended to follow the manufacturer’s instructions and to ensure proper application conditions such as stirring, temperature, humidity, and ventilation. Additionally, ensure the wooden surface, where it is being applied, is dry (oil free), sanded and contamination-free.
Polyurethane should dry properly to ensure that it fully cures and reaches its intended hardness and durability. By allowing polyurethane to dry properly, you can ensure that it provides the desired protection and appearance to the surface it is applied to.