Several months ago, a client of mine asked me whether he could apply second or multiple coats of polyurethane without sanding. So, I obligingly told him my take about polyurethane coating and the steps when applying polyurethane coats numerous times. Polyurethane, of course, needs no thinning, nor does it require any unique substance to enable it to bond with the wood. Moreover, it is easy to sand sans clogging the sandpaper.
One quality of polyurethane you need to take note of is its slow-drying property. Thus, you will more often see vestiges of dust on it that need sanding before applying the next coat. Yet, even if you don’t sand between coats, it doesn’t make a big impact on the result. Nevertheless, it will be best to sand before applying the next coat to quicken the adhesion process between layers to create a leveled finish.
When Would You Not Need to Sand Between Polyurethane Coats?
Woodworkers call the process of applying the second polyurethane coat over the first sans sanding as a hot coating. It is an accepted technique, of course. It also appears to be very effective if you do it immediately after the first coat has already dried. This good result may be because the substance is still polymerizing. It will be best after the second coat to let the surface cure. Once cured, it is advisable to sand a bit before applying another polyurethane coat. Then another hot coating.
Sanding, of course, doesn’t mean that you need to sand vigorously. When sanding, only rub the surface a bit, which means to sand it enough to make the surface feel a bit rough for the next layer to lock in. In this way, the coat will not peel away. A light swipe of the sanding paper may do to get a perfect result.
Why Is Sanding Between Coats Advisable?
As mentioned above, polyurethane is slow drying. As such, it may accumulate dust and debris during its long drying process. Thus, when applying multiple coats of urethane, if dust settles on the coat, it will get enhanced and amplified as you apply another coat.
If you’re a perfectionist who wants a perfect finish for your polyurethane coats, it will be best to wait for the coat to dry up patiently. Then, use sandpaper with 220 or finer grit. Such sandpaper will remove even the dust bumps.
Afterward, using a slightly damp cloth, you can wipe the surface clean and buff it dry using a dry cloth. Only then should you apply the next coat. I would advise you to refrain from using any mineral spirit.
Another reason why you should sand in between polyurethane coats is because you can cover up any flaw when you sand. Sanding may not be necessary, but it will no doubt facilitate the covering up of any application flaw.
Application flaws may occur if you are not used to varnishing or brushing. You may end up with sloppy brushstrokes that end up with uneven applications. You may overlook these flaws when applying several coats, but they may become evident once the coats have dried up. So, to solve such an issue, you can sand in between coats to ensure that you will get better results once you’re done coating.
Caveats When Coating Wood with Polyurethane
Multiple coats of polyurethane will indeed produce excellent results if you know how to do it right. In my case, I would sometimes apply three to six coats on the wood. But for every coat, I need to wait for the coat to completely dry. Then, I sand it and wipe it with a dry rag before I apply another coat. Such a process may take several days to accomplish, so it requires great patience.
One caveat, however, that you need to bear in mind is that there is a strong likelihood that the finish will be dull once you sand the topcoat more. As a tip, you can make the finish glossier if you use super-high grit sandpaper.
One more thing is that this advice is best only for oil-based polyurethane as well as if the wood is tight-grained. If open pores characterize the wood, for example, you might as well apply other techniques to come out with a smoother surface.
Steps on Applying Multiple Polyurethane Coats
If you are still a bit tentative as to whether you are doing the right thing or not when applying multiple polyurethane coats, you can follow the following simple steps on applying multiple polyurethane coats:
Step 1: Apply the First Coat
The primary step is to prep the wood before applying the first polyurethane coat. Thin it out with a paint thinner. In this way, it will dry faster, allowing you to sand its surface soon. Let it dry and cure for a day before sanding. It will be best to use fine-grit abrasive sandpaper and sand along the wood grain. Then, remove the dust first using a static duster and a tack cloth. In-between Coats
Step 2: Apply the Second Coat
After the first coat had dried and cured, you can apply two or more additional coats to the surface. You can sand between each coat. While you apply polyurethane, you can start by brushing along the grain. Try to stretch the polyurethane thinly as possible. Let every coat dry for one to two days before you sand.
Step 3: Apply the Final Coat
As a caveat, you should not sand the final coat of polyurethane. Let the coat harden. Once it is hard enough, you can apply paste wax or other protective furniture wax to give it its final protection.
Step 4: Finished Wood Surface Maintenance (Optional)
To maintain your finished wood surface, you can apply a paste wax coat by simply rubbing the paste wax on the wood using a soft cloth. When applying the paste wax, use a circular motion. Make sure that you apply the wax on all parts of the wood and let it be for half an hour. Then, buff the wood surface using a clean and soft cloth.
You can apply up to three coats using the same circular motion. It will also help to regularly dust the finished wood surface using a soft cloth or a feather duster. You can do this maintenance every six months to a year, depending on how frequently you use the furniture or surface.
Frequently Asked Questions
Aside from the abovementioned ideas and concepts about the use of polyurethane, it will be useful to know the following FAQs about the use of polyurethane:
Is It Okay to Apply Over Stain a Polyurethane Coating?
You can apply polyurethane over a stained surface. Nevertheless, it will help to consider that the water-based polyurethane may not combine well with oil-based stains. As such, you need to sand the surface first before you apply water-based polyurethane on an oil-based stain. You will need rough sandpaper to make the surface rough enough for the water-based polyurethane to jive well with the surface.
Is It Enough to Only Apply 2 Polyurethane Coatings?
Yes, for some instances, it will be enough. But the more coats you apply, the thicker, glossier, and stronger the finish would be. So, if you want to ensure that the protective coating will last longer, apply more coatings, for it will be better for your wood surface. But I would repeat, in some instances, two layers may be enough.
The polyurethane coating may look visually similar as compared to other coatings like epoxy, for example. Yet, it carries several unique properties. It is durable, more elastic, and softer. Hence, it is ideal for areas that see heavy pedestrian traffic. Besides, it resists more abrasion and scratches.
Moreover, it can maintain its shape more in lower temperatures. These characteristics make polyurethane perfect for various applications. Plus, some water-based formulated polyurethane coatings exhibit a non-flammable characteristic.
Many coatings of polyurethane necessitate no additional processing to achieve maximum curing. You simply need to apply them and let them cure once exposed to air moisture. Compared to epoxy coatings, it cures quickly and can be ready for use within 24 hours. It also exhibits better resistance to substances like lactic acids, making it perfect for floor coatings in many industrial plants. Lastly, it adheres well to various types of substrates.
Jason is a 40-year-old woodworker, carpenter and author who have been involved in the woodworking and woodcraft industry with 17 years of experience. He is expertise in technical aspects, woodcraft and furniture building projects.