how long does polyurethane take to dry

How Long Does Polyurethane Take To Dry

Most likely you’re finishing a floor or varnishing some furniture.  You have at least three more coats to apply and a million things you have to take care of, and you just applied the first coat!  How long is this going to take? Like most simple questions, the answer is a bit more complex. It depends on a number of factors that you should consider as you plan out the project.  The principal variables are what kind of polyurethane are you using and how warm the room is that you’re working in. 

So, how long does it take for polyurethane to dry?  It takes oil-based polyurethane 24 hours and water-based polyurethane 6 hours to dry enough to be able to walk on it in socks and proceed to the next step of the application process.  The next step may be sanding or applying the next coat. Each of these steps again requires the polyurethane to be dry.

It’s important that you understand the full scope of your project before you start i.e. how many coats of polyurethane do you plan to apply, or do you expect to sand the floor? 

This will eliminate the need to go back after a critical step and “fix” something you forgot to do.  You can use these “rules of thumb” to plan your project:

Oil Based Polyurethane Drying Time and Curing Time 

  • After 24 hours, you can walk on the floors.  The floor should “look” dry and not be tacky to the touch. Keep pets off and walk on the floors wearing socks.  Don’t wear shoes and don’t walk in bare feet.  
    • Sanding can be done now if you intend to.  
    • The next coat can be added now if you intend to apply another coat.
  • 48 hours can walk on floor with shoes
  • After 4 days, move furniture back
  • Keep animals off for 1-2 weeks
  • 30 days for full cure.  Leave floor uncovered at least 2 weeks. 

Water-Based Polyurethane Drying Time and Curing Time 

  • 4-6 hours before you can walk on the floors.  
  • After 6 hours, the floor should “look” dry and not be tacky to the touch. Keep pets off and walk on the floors wearing socks.  Don’t wear shoes and don’t walk in bare feet.  
    • Sanding can be done now if you intend to.  
    • The next coat can be added now if you intend to apply another coat.
  • After 24 hours can walk on the floor with shoes
  • After 2 days, move furniture back
  • Keep animals off for 1 week
  • 30 days for full cure.  Leave floor uncovered at least 2 weeks. 

Polyurethane Drying vs. Curing

There are two phases to applying a polyurethane finish to a floor.  Drying time is the recommended duration needed between coats. Curing time is the recommended length of time before using it.  Keep in mind that each manufacturer produces a range of finishes each with a secret recipe with different additives and chemicals affecting the application, the behavior of the product while you’re applying it, and the result.  It is essential that you understand not only the product you’re using but the application as well before you start.

Drying Polyurethane

The first phase is the “drying” phase.  When you apply polyurethane to the floor, you’re applying a layer of polyurethane resin in a liquid solvent, to the floor.  The resin is what remains after the solvent dries and is the material that forms the final barrier protecting the wood. The solvent can be either water or some type of finishing the oil, e.g. linseed, tung, or walnut oils.  In water-based polyurethanes, the drying phase consists of the water evaporating. In oil-based polyurethanes, the finishing oils evaporate. Water simply dries faster than the oils or solvents.

Curing Polyurethane

The second phase is the “curing” phase.  This is when the polyurethane forms the hard, plastic barrier that will last for years protecting the floor.  Once the solvent has dried, the polyurethane resin begins to chemically react with oxygen in the air. The oxygen reacts with the polyurethane crosslinking the polymer molecules and forming a three-dimensional matrix of the resin.  

Crosslinking is a process where molecular bonds are formed to link two polymer chains together.  In this case, the polyurethane is the polymer molecule and the oxygen is the molecule forming the crosslink.  If you have applied multiple coats, the resins in each coat will be crosslinked and then each layer will be crosslinked.  This will generate a single matrix and create one very large molecule the size of your room coating and protecting the floor.  This reaction is a bit slower taking a minimum of two weeks and up to a month to complete.  

Factors affecting polyurethane drying and curing

Every project is unique.  The length of this type of project is measured in days and weeks instead of minutes and hours.  When you’re planning how you’re going to approach your project, keep in mind that the “rules of thumb” listed above are affected by a host of variables:

Type of Wood

Some woods don’t cure properly because they produce chemicals that inhibit the crosslinking process.  Rosewoods and some aromatic cedars are included in this category.  

 Type of Polyurethane

Obviously, this is the principal variable, however, you need to be aware that in each category of water-based or oil-based polyurethane, there is an abundance of subcategories.  Some manufacturers add drying agents. Some add oil and possibly other solvents. You can get stains to add color and the list goes on. Each component may impact the dry and cure time by a little or it may force you to change your entire approach.  Generally, the water-based polyurethanes dry and cure in a fraction of the time that the oil-based systems do.

Temperature and Humidity

The times stated above are based on a 70°F (21°C) day and 70% humidity.  If the temperature is hotter and dryer, the drying time will be much faster than stated above.  Conversely, a cooler temperature on a wet day could extend the drying time by as much as half a day.

Starting Surface

Raw, sanded wood will absorb the first coat and will shorten the dry time.  The first coat wets and seals the wood. Each subsequent coat consists of applying one layer over an existing layer of polyurethane and will take longer to dry.  

Mixing the type of polyurethanes

Mixing polyurethanes is complex.  Remember you’re trying to form a single large molecule.  If you are using one type of polyurethane with the same kind of solvent or oil, the chemical reaction turns out to be straightforward without a lot of mitigating factors.  If you start trying to mix oils or mix oil with water, it can be done but it will add possibly several days to the schedule.

Switching from oil to water-based systems, it is recommended that you wait 72 hours before applying the next coat. Switching from water to oil-based systems, it is recommended waiting only 24 hours. The point of waiting is to ensure the first coat is completely dry before introducing a solvent that may not be compatible with the previous coat.

How it’s applied

If you apply the polyurethane by brush, the layers will be thicker than if you spray it on or if you’re applying it and then wiping it off (a technique used in woodworking).  

Polyurethane Safety

So, is using polyurethane safe? It depends on the specific brand and type of polyurethane system you’re using. Before using ANY chemical, read the instructions and pay particular attention to the safety precautions!  Here are some more rules of thumb:

  • While the polyurethane is drying and curing, oil-based systems smell a lot worse and contain chemicals in the solvent that can be harmful to your respiratory system.  Water-based polyurethane doesn’t smell as bad as the oil-based systems, but the cautions still apply. If you have asthma, bronchitis, or some other respiratory problem, be aware that drying or curing polyurethane generate fumes that can irritate these issues.  Once the floor is fully cured, there is no further cause for concern.  
  • Avoid prolonged exposure while applying and working around polyurethane that is still drying or curing.  Make sure the area is well ventilated. Follow directions.
  • Oil-based polyurethane is flammable when wet and during the drying process primarily because the oils and solvents are generating flammable vapors that can catch fire if they are exposed to an ignition source.  This applies to wet rags used to wipe up any liquids. Water-based systems are not flammable. 
  • Avoid smoking or activities that could potentially generate sparks.  If using oil-based polyurethane, consider having a fire extinguisher handy.  Always follow directions.

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