Polyurethane is a flexible, durable, and moisture-resistant sealer used for different surfaces, such as wood, plastic, and metal. So, should you sand between coats of it?
Yes, you should sand between coats of polyurethane to prevent dust build-up, brush marks, or imperfections to appear once the last coat dries.
If you skip sanding between coats of polyurethane, the subsequent coats won’t bond properly to the previous coat. Dust nibs and grain would settle over the coating, preventing a good adhesion and an even finish.
You should sand before applying polyurethane to remove dust, dirt, or imperfections from the surface. However, you shouldn’t sand the final coat of polyurethane as it removes its glossy, protective, and moisture-resistant layer and exposes the surface to weather elements.
Why Should You Sand Between Polyurethane Coats?
You should sand between coats of polyurethane for the following reasons.
- Removes dust and debris.
- Removes brush marks.
- Removes imperfections.
- Improves adhesion between coats.
- Enhances smoothness.
- Ensures long-term durability.
- Improves finish quality.
If a surface has dust, debris, brush marks, or imperfections, it will prevent subsequent coats to adhere properly and result in a rough and uneven finish. To improve adhesion between coats and get a smooth and fine finish, the surface and previous coats have to be smooth and fine too.
Polyurethane is colorless, so any brush stroke mistake or unaddressed dust nibs become noticeable once the sealer dries. So, if you don’t sand the coat after it dries, it means the next coat will be applied over dust and brush marks, resulting in an uneven finish.
Also, polyurethane’s glossy finish repels moisture (liquid). Therefore, sanding off its glossy and moisture-resistant layer between coats is mandatory or the next coating won’t adhere.
What Happens If You Don’t Sand Between Coats of Polyurethane?
If you don’t sand between coats of polyurethane, the finish will be uneven, sticky, and blurry, and it will peel off. The coatings will have a weak adhesion, visible brush marks, lower durability, and will take longer to dry.
This is because dust or dirt trapped on the existing coat will prevent the next coating to adhere properly. Since the coating won’t adhere properly, it will remain wet longer, turn sticky, and will peel off.
Since polyurethane is a colorless finish, it is susceptible to brush marks. So, if you don’t sand between coats, brush marks will be visible after you apply the subsequent coatings.
Moreover, polyurethane has a glossy moisture-resistant finish that doesn’t allow moisture (or liquids) to penetrate it. So, if you don’t sand the existing coat, the glossy moisture-resistant layer won’t allow the next polyurethane coating to stick.
Should You Sand Before Applying Polyurethane?
You should sand before applying the polyurethane. Sanding is necessary because polyurethane is a clear finish (colorless), and without sanding, the imperfections on the surface become visible after the sealer dries.
Sanding before applying polyurethane is done to remove blemishes, pencil marks, glue, and bumps from the surface. These imperfections can impact the application, smoothness, and drying time of the sealer.
To sand bare wood surfaces before poly, use medium-grit sandpaper (120/150-grit), and finish with fine-grit sandpaper (120/220-grit). You must sand wood surfaces as they have pencil marks, glue residue, and dents left by the carpenter, which can prevent good adhesion and result in an uneven finish.
It’s also important to sand if the surface is already finished with another sealant. By sanding, you remove the existing finish and imperfections from the surface, leaving behind a bare and smooth surface. This is an important step, as polyurethane needs a clean and well-prepared surface to adhere to.
If the surface is finished with a strong sealant, such as varnish, start sanding it with coarse-grit sandpaper (40-grit). If the surface is finished with paint (and you want to remove it), start sanding with medium-grit (100-grit) sandpaper and progress to fine-grit sandpaper (220-grit).
Should You Sand Between Polyurethane Coats?
You should sand between polyurethane coats. Sanding polyurethane differs from regular sanding as it focuses on removing imperfections and abrade the previous coat, rather than removing the entire finish or glossy layer.
To sand between coats of polyurethane, use fine-grit sandpaper (120/220-grit) as the sealer isn’t thick. Using medium or coarse-grit sandpaper (100 or 40-grit) may remove the entire finish.
Simply swipe the fine-grit sandpaper (120/220-grit) over the coating until the imperfections are removed. If the coating has minimal imperfections, use extra-fine grit sandpaper (440-grit) and don’t sand as much.
While sanding, it’s important to follow a proper technique to get better results. Apply light to moderate pressure, and avoid pressing the sandpaper too much as that can remove some parts of the finish. Always sand in the direction of the grain.
After sanding each coat, remove dust particles or sanding residue using a clean lint-free cloth. This ensures that the coating is clean and improves the adhesion when the next coating is applied.
Should You Sand The Final Coating?
You shouldn’t sand the final coating of polyurethane as it removes its glossy, protective, UV-resistant, and moisture-resistant layer, exposing the surface underneath to weather elements.
The purpose of polyurethane is to protect the surface (or finish) underneath by producing a flexible, glossy, UV-resistant, and moisture-resistant coating. This coating prevents water (and other liquids) from reaching the surface (or finish) underneath.
You will remove this protective barrier and expose the surface to weather elements if you sand the final coating. Basically, it defeats the purpose of the sealer.
However, if the final coating has an imperfection, sand it with extra fine-grit sandpaper (440-grit). The extra fine-grit sandpaper will remove the imperfections, but won’t remove the glossy protective barrier of the sealer.
Related Read: Should You Sand Coats of Stain?
How Long To Wait Before Sanding?
You must wait until the polyurethane coating fully dries (24-48 hours) before sanding it. On average, it’s recommended to wait 24 hours before sanding a polyurethane coating. The drying time depends on factors such as the sealer type, room temperature, humidity levels, and surface type.
For example, water-based polyurethane typically cures within 24 hours due to its water-based solvent. While oil-based polyurethane may take up to 48 hours to fully dry (cure) due to the slower evaporation rate of oil. Room temperature, humidity levels, and thickness of the coat also determine the drying time.
For the sealer to dry, its solvent (water or oil) must evaporate from the existing coating. Once the solvent evaporates, the coating begins to oxidize and harden.
Once the coating has hardened and fully dries, you can sand it. Since polyurethane is a thin finish, it’s crucial to allow its coating to dry fully to avoid damaging it.
Are There Polyurethane Types That You Don’t Need To Sand?
Yes, you don’t need to sand fast-drying polyurethane. Since fast-drying polyurethane dries fast and doesn’t stay sticky (or wet) for too long, you don’t have to sand between coats. The longer a coat stays sticky (or wet), the more dust or dirt it will attract.
However, if the coating dries fast and doesn’t stay wet for too long, the dust or dirt that it will attract will be minimal. So, instead of sanding fast-drying polyurethane, use a clean cloth to wipe it before applying the next coating.
According to Minxwax, a leading brand for producing polyurethanes and wood stains, fast-drying polyurethane takes only 4-6 hours to dry between coats.
How Many Coats of Polyurethane Do You Need?
You need 2-5 coats of polyurethane on a surface for proper protection. If the surface doesn’t need much protection, apply 2-3 coats. If the surface is placed outdoors and needs more protection, apply 5 coats.
The minimum number of coats of polyurethane is 2, while the maximum is 5 coats. If you apply less than 2 coats or more than 5 coats, the finish won’t be smooth, it won’t protect the surface properly, and it can turn sticky (or remain wet).
Can You Re-coat Polyurethane Without Sanding?
You can’t re-coat polyurethane without sanding if you want a smooth and durable finish. However, you can use steel wool instead of sandpaper.
Steel wool, an abrasive material used for burnishing, can remove imperfections from a coating before you apply the next one. It’s recommended to use 0000-grit steel wool between coats of polyurethane.
You can apply one coat of polyurethane if you don’t want to sand a surface. However, one coat isn’t enough to protect a surface.