There comes a time when you think you have done everything right, the polyurethane won’t dry, so you’re left scratching your head wondering if you used an expired polyurethane.
And obviously, it’s not like a piece of machinery that you can take apart to locate the problem. No, because that would mean ruining your job.
So, what are your options?
To figure out what to do when polyurethane won’t completely dry, you must first understand the different factors that affect drying time and how to counter each one.
How Long Should Polyurethane Take to Dry?
Polyurethane can take anywhere from 2 to 24 hours to dry.
Water-based polyurethane gets completely dry and ready for the second coat in 2-3 hours, while oil-based polyurethane needs 6-24 hours.
However, it takes 21 days for water-based polyurethane to cure and 30 days for oil-based polyurethane.
How Can You Tell When Polyurethane Has Dried?
The two types of polyurethane have different features when dry:
- Oil-based polyurethane no longer has a smell, and it is not tacky to the touch.
- Water-based polyurethane stops being cool to the touch.
After the last coat, water-based polyurethane will dry in 24 hours and can be ready for light use in 3 days. On the other hand, oil-based polyurethane takes about 72 hours to dry and at least 7 days before light use.
If it has gone beyond these curing time frames and the polyurethane still isn’t dry, then you have a problem.
Related read: What is the ideal interval between coats of polyurethane on hardwood floors?
Factors Affecting the Drying Time of Polyurethane
Here are the factors that cause polyurethane not drying:
1. Type of Polyurethane
The first factor that determines how quickly polyurethane dries is the type of base. Oil-based polyurethanes dry much slower than water-based polyurethane.
If you apply water-based polyurethane over oil-based polyurethane, it will take considerably longer to dry. What is worse is if you applied subsequent finishing coats when the previous one dint dry properly.
In that case, you would have created one thick coat instead of three thin ones.
To speed up the drying process, you should thin the polyurethane. Thinning polyurethane will help you apply thin coats that evaporate quickly. However, this isn’t a problem you can fix after application unless you want to strip it and start all over again.
2. The Product You are Using
Not all polyurethanes are equal. Even within the same company, you can find various drying times. For example, Minwax ultra-fast drying polyurethane for wooden floor dries in 2 hours, while the Minwax fast-drying polyurethane needs 4-6 hours to dry.
When using polyurethane for a while, you might have gotten into a habit of not reading the manufacturer’s instructions. Instead, you go ahead and apply one formula precisely the same as you would another and expect the same results.
Even when they are made by the same company, the drying process can be vastly different.
Besides that, you need to ensure you use the right polyurethane finish for the right surface. For example, some polyurethanes are explicitly meant for wooden floor, meaning you should not use them for anything else.
In the same way, some polyurethanes are for exterior use, while some are for interior wood surfaces. If you have used the wrong product, you need to strip it off entirely and use the right one.
However, if you followed the instructions correctly and exceeded the curing time by several hours, it’s time to consider other factors.
Besides the type of polyurethane and the brand you are using, heat is usually the next most important factor. After all, it is warm air that makes the excess fluids evaporate from the polyurethane.
If you miss applying polyurethane in the suggested temperature range, you could end up with drastically different results.
The ideal temperature for polyurethane to dry is between 70 – 77 degrees Fahrenheit. If the temperature is much lower than this, it can extend the drying time by a few hours or dry overnight.
When the temperature is below 50°F, many manufacturers suggest you shouldn’t bother trying to apply polyurethane because it won’t dry.
The good news is that the temperature is something you can control. If you have an HVAC system in your home or at the site, check to see if the thermostat is in the right setting and adjust accordingly.
Don’t overcompensate by cranking the heat up too high. If the room is too hot, the polyurethane will evaporate too quickly, and it might crack.
Check the instructions to ensure the temperature is set correctly. Another way to boost the temperature is to use a blow dryer, heat lamps, or hair dryers. To ensure that you maintain the right temperature while applying heat, use a thermometer.
If the room temperature has been just fine, another possible culprit to investigate is the humidity. When it is too humid, polyurethane takes longer to dry because moisture is constantly being introduced to the polyurethane finish.
But, as you would expect, the opposite is also true. The ideal relative humidity (RH) level for water-based polyurethane to dry is 50% and 70% for oil-based polyurethane. But, of course, brands and products differ.
You can measure humidity with a hygrometer. If the RH level is too low, meaning it is not humid enough, use a humidifier to boost it. If it is too humid, use a dehumidifier.
Of course, the ideal situation is a thermostat that also allows you to set the humidity level.
The last major factor that affects the drying time of polyurethane is ventilation. Just like with the temperature, ventilation allows the poly finish to evaporate quicker.
As it lets out the odor, it also reduces the humidity in the room and ushers in fresh air that isn’t saturated. If you had previously shut the windows or the doors to prevent dust from coming in or you forgot, it’s time to open them up.
Proper ventilation can speed up the drying time by several hours, depending on how stuffy the room is. On a separate note, ventilation is also good for your health as it allows the toxic gases to escape.
What if it Still Hasn’t Dried After Trying These Solutions?
If you have given these different solutions several hours to work and there is still no difference, it might be time to consider stripping the polyurethane and starting all over.
I should mention that it is rare for a coat of polyurethane not to dry unless it is really thick. If it isn’t too thick and you have investigated all of these options, then you might have gotten a defective product.
However, ensure you give these solutions sufficient curing time to work before you accuse a manufacturer falsely.
How to Fix Sticky Polyurethane
Even after you have done all there is to do; polyurethane might still end up sticky. While this is rare, it is helpful to know how to fix sticky finish.
If you have patiently given it enough time to dry and cure – 21 days for water-based polyurethane and 30 days for oil-based polyurethane finish– then it might be time to strip and start all over.
How to Work Safely When Striping Polyurethane
Given how dangerous polyurethane and paint strippers can be, it is important to work cautiously when trying to fix sticky polyurethane. But is polyurethane toxic after it dries?
- The first thing you need to do is work in a well-ventilated area. Crack the windows open and get a fan to help with the fumes. If it isn’t windy, it is preferable to apply the paint stripper outside. Once you have taken the polyurethane off, you can bring it indoors for reapplication.
- The second thing you should do is wear personal protective equipment. Depending on the type of stripper you are using, you may need to cover your body completely to prevent the chemicals from interacting with your skin, eyes, mouth or nose.
- Finally, get all of your tools together in the working area and shut it securely. The last thing you need it a child or a pet wandering in when working with hazardous chemicals.
- Once you have protected yourself and others from any potential harm, it is time to get to work.
Tools Needed to Strip Polyurethane
- Paint stripper
- Paint brush
- Lacquer thinner
- Plastic scraper
- Rubber gloves
- Respirator mask
- Cloth or paper towel
- Floor coverings
How to Fix Sticky Polyurethane – Step by Step
Step 1: Put on Your Personal Protective Equipment
Wear your goggles, gloves, and respirator. When you watch some videos on YouTube, you might see some people without gloves or goggles, but that is a bad habit that could spell disaster.
As mentioned earlier, paint stripper is a powerful chemical, so you don’t want it to reach anywhere sensitive. You may also choose to wear overalls.
Step 2: Apply the Paint Stripper
Using an old brush, apply a generous amount of the paint stripper to the polyurethane. You have to make sure the finished surface is coated properly, so the paint stripper does its job effectively.
Once you have used a brush to apply paint stripper, don’t use the same brush to apply polyurethane. Unless, of course, to apply paint stripper later on, but it’s best to dispose of it.
Read also: How to clean polyurethane brush.
Step 3: Give the Stripper Time to Work
The stripper will need to sit for between 10 to 15 minutes. This will give it enough time to penetrate the top coat of the polyurethane and begin to break it down.
Step 4: Scrape off the Polyurethane
When you have waited a while, you will see the polyurethane bubble and begin to peel. Use a plastic scraper to get rid of the loose polyurethane. You can apply a bit of pressure because the plastic won’t damage the wood.
Step 5: Wipe off the Remaining Polyurethane
Use a damp cloth or scrub pad (also known as a scouring pad) to remove polyurethane. If the cloth is getting dry or clogged, dip it in water and keep stripping the polyurethane.
If there is still any polyurethane left after this, repeat steps 2 to 5 until all you’re left with is the bare wood or underlying stain.
Read also: What is the best way to remove polyurethane from wood?
Can I Sand Sticky Polyurethane?
No, it would be best if you did not sand sticky polyurethane. If you try to sand it, the wet polyurethane will gum up the sandpaper and leave the work uneven. Instead, you should leave the polyurethane to dry before sanding.
Can I Apply Another Coat of Polyurethane When the First One isn’t Dry?
You should not apply another coat of polyurethane over wet polyurethane. Otherwise, you will be left with a thick coat, which will take much longer to dry if it even dries. Another reason to let the first coat dry is so you can correct any problems quickly.
Is it Safe to Use Paint Strippers Indoors?
Yes, you can use paint strippers indoors, provided you follow all safety precautions. You must work in a well-ventilated area and get a fan to blow the fumes away from you towards the doors. Also, wear protective goggles, gloves, and a respirator.
Can you Paint Over Polyurethane?
Yes, you can paint over polyurethane once the curing process is over. You would need to sand the polyurethane so that the paint can be absorbed into the wood.
Related: Can you put polyurethane over paint?
Polyurethane Still Feels Sticky After 48 Hours
Polyurethane is usually dry to the touch between 2 – 12 hours. If polyurethane still feels sticky after 48 hours, it could be that the wood has natural oil preventing the polyurethane from drying. Once the first coat dries, the subsequent polyurethane coats should be fine.
How to Remove Sticky Polyurethane
You can remove sticky polyurethane without sanding by using a paint stripper. Apply stripper generously, let it sit for ten minutes, then use a plastic scraper to take it off. If you try to use sandpaper to remove sticky polyurethane, it will gum up.
What to do When Polyurethane Won’t Dry – Conclusion
It can be frustrating not knowing what to do when polyurethane won’t dry completely, especially if the project is for a client and you are way behind schedule.
If this is something you are struggling with, I hope you have found this article helpful, and I’m sure you’ll find an ultimate solution here.
One last tip – if you are applying polyurethane on stain, always give it enough time to dry naturally, or it will seriously slow down your work.
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