Can I apply a second coat of polyurethane without sanding?
Yes. But it isn’t recommended. While polyurethane dries – somewhere between 6 and 24 hours – it becomes tacky before it finally hardens, trapping dust, pet hair, and other particles during that time. Along with grain raising that can sometimes occur, you could be left with disappointing results if you don’t sand between coats.
Polyurethane is meant to smooth and protect wood surfaces, but if it is applied incorrectly, its benefits can be completely nullified. Sanding is an important part of that application process. You may be able to get by without it, but you are not guaranteed the same long-lasting results you would get with industry-wide recommended applications.
What is Polyurethane?
Polyurethane is used to coat the wood in order to protect it from moisture, mold, and stains. Surfaces within your home that need to be sealed include floors, furniture, cabinets, decks, and porches.
Most of these sealers do not actually contain pure urethane, but rather they utilize urethane polyether or polyester fortified with curing agents.
Just like paint, you can purchase polyurethane in a variety of sheens, including high gloss, semi-gloss, and satin.
Oil-based or Acrylic Polyurethane
Polyurethane comes in oil-based and water-based varieties, just like paint. Here are a few pros and cons about each type.
Acrylic (water-based) urethane
The main concern with water-based sealants is that they can cause grain raising due to the moisture in the product or even humidity in the air.
(Grain raising is what happens when the small wood fibers absorb this moisture and stick up, making the wood feel rough and appear dull after a coat of poly. The next coat won’t go on smooth, especially if you don’t sand the first coat down.)
A few other details about acrylic polys:
- Low- or no-odor – can be used in a poorly ventilated area
- Dry faster – finish your overall project faster
- More expensive – surprisingly, these products tend to be higher priced than traditional polyurethane
Traditional (oil-based) polyurethane
Just like oil-based paints, traditional polyurethane products are harder to clean off if you get them on an unintended surface, but they are generally preferred because they are less likely to raise the wood grain.
A couple of things to consider about oil-based polys:
- Longer lasting – the results may look the same initially, but over time, oil-based polys hold up better than water-based ones
- Strong odor – the unmistakable odor of these varnishes require excellent ventilation
The type of wood you are sealing may dictate which type of polyurethane you need. Porous woods like pine, for example, need to be sealed with oil-based polyurethane because they have a tendency to grain-raise, but hardwoods don’t have this problem quite as frequently and can, therefore, tolerate either type fairly well.
Thinning Varnish Prior to Application
Woodworking experts sometimes choose to thin their polyurethane products. As with full-strength urethane, this has its own set of advantages and disadvantages.
You can make your own thinned varnish by mixing equal parts poly and mineral spirits. You can also purchase thinner urethane products. They will usually be labeled “wipe-on,” and they tend to be more expensive than full-strength polyurethanes.
- Thinned polyurethane can be wiped on, so there are no brushes to clean. Just wait until the cloths are dry, then throw them.
- Dries faster between coats.
- Offers better self-leveling results.
- Thinner products will require more coats. Don’t be surprised if you have to apply up to 3 thin coats for every one coat you would normally do with full strength. You may end up applying 3-6 coats of thinned poly versus 2-3 coats with an unmixed product.
- Because you will have more layers, you will create more work for yourself by sanding between multiple coats.
How to Apply Polyurethane
It is necessary to be familiar with the application methods of polyurethane in order to understand why sanding is an important part of the process.
Method 1: Most Recommended
Practiced by professionals. Widely accepted as giving the best results.
Before the first coat of poly, you will need to clean and sand the surface of the wood. Get rid of all dust particles and sanding debris using a vacuum and then a damp cloth. (Side note: New polyurethane can be applied over an older coat as long as the surface is sanded down and cleaned.)
Once the wood is completely dry, apply the first coat of polyurethane using a wipe-on or brush-on formula.
These preparatory steps will be the same regardless of the method you proceed with.
This is where the methods differ:
Wait until the first coat is completely dry. This will take at least 6 hours for water-based sealants, but most pros recommend waiting a full 24 hours. Then sand very lightly with fine-grain sandpaper to remove dust or anything else that has settled on the sticky surface during drying.
Pro tip: You should only see fine dust from sanding. If you see gooey balls forming, your previous coat is not dry.
Wipe the wood clean with a damp cloth after sanding. Tack cloth is recommended for this step because it picks up the tiniest particles.
Now you will apply your second coat of polyurethane. Let the surface dry completely before applying the next coat.
If you wish to add a third coat, repeat the same steps.
The difference between this and other methods is sanding between each layer.
Dust particles that remain between layers of polyurethane can easily be knocked off, taking other layers with it – since they are bonded together – exposing the bare wood. This allows moisture to come in contact with the bare wood, which can discolor it or cause it to swell.
Method 2: No Sanding Required
Thought to provide better adhesion. Faster.
This polyurethane application method begins the same as our first example:
- Clean the surface
- Sand the bare wood gently
- Apply first coat by your preferred method
And here is where it differs:
Without sanding the first coat, apply a second coat less than two hours after the first.
This creates a chemical bond between the two layers. However, it offers no protection against the particles that get stuck to the first layer. Sanding is not an option after only 2 hours because the product will still be sticky.
When debating the “right way” and “wrong way” to apply polyurethane, it’s important to point out that both of these methods are listed on the packaging of many polyurethane products, so both are considered acceptable by manufacturers’ standards.
Method 3: Best of Both Worlds
Minimal sanding. Comparably smooth finish.
If you will be applying 3 or more coats of polyurethane, you may want to consider this hybrid application technique.
For this method, you’ll prep the surface as normal.
Then, follow the steps for method 2, applying layers 2 hours apart until the last coat.
Then, wait at least 24 hours between the next to last and last coat, sanding before the last layer only to get a smoother finish without quite as much sanding.
Tips for Non-Sanding Methods
If you have weighed the options and you simply don’t want the extra work of sanding between layers of polyurethane, take our advice for getting the smoothest finish possible without sanding.
Apply thin layers. You want your product to dry as quickly as possible so it doesn’t attract more particles. Using thinned polyurethane or a wipe-on product will result in the thinnest, quickest drying layers.
Observe longer wait times between coats. Check the manufacturer’s recommendations, but we recommend at least 6 hours between coats for water-based and 24 hours minimum for oil-based polyurethanes.
If you do choose a method that requires sanding, here are a few guidelines to follow:
Sand with 220 grit sandpaper after the first coat. Move to 300 or higher after the next coat. After the final coat, if you have been careful to maintain a wet edge (as in painting) then you should not have to sand the final coat.
However, if you see flaws in the topcoat, or if you just want the smoothest finish possible, follow the final coat of polyurethane with a very light sanding. Some pros recommend 600+ grain, but it isn’t uncommon to see woodworkers who use 1500 grain on the last coat for a glass-like finish.
You can also use 00 (fine) to 0000 (superfine) grade steel wool instead of sandpaper. Some people find it easier to work with and its results more predictable than sandpaper.
When applying sealant, two coats is sufficient for the vast majority of projects, but you can do more or less depending on your preference.
Commercial bar tops and floors in high traffic areas can take up to 3 coats, but much more than that is going to result in uneven product and actually results in more likelihood that it will peel.
Following the grain isn’t necessary with poly, but most experienced woodworkers will do it anyway to ensure the very best results.