When applying polyurethane on a surface, you should sand between coats of the finish. But you may be wondering – what happens if you don’t sand between coats of it?
If you don’t sand between coats of polyurethane, the next coats that you apply wouldn’t bond properly to the previous coat. The finish will also be uneven and rough because of dust nibs and grains that settle on the coating while it’s drying.
Why Should You Sand?
The main reason to sand between coats of polyurethane is to get rid of dust, brush marks, and imperfections on the surface. These things appear on the coating after it has dried. If these imperfections are left on the surface, the next coats won’t stick properly. The overall finish will also come out rough, uneven, and ugly.
To get a smooth and fine finish, the surface and previous coats have to be smooth and fine.
Since polyurethane is thin, any mistake with the brush stroke with be noticed once the top coat dries. Dust will also settle over the coating. So, if you don’t sand it after it dries, the next coat will be applied over that dust and brush marks. Because of that, the finish won’t come smooth and fine.
Also, polyurethane has a glossy finish that repels moisture (liquid). So, if you don’t sand the glossy layer between the coats, the new coating won’t stick over it.
Dry Time Before Sanding
You have to wait until the polyurethane dries fully (cures) before sanding it. On average, you have to wait 24 hours after applying it before you can sand the finish.
For the sealant to dry, the solvent (water or oil) must evaporate from the coating. Once the solvent evaporates, the coating will start to oxidize and harden.
The dry time of the sealant depends on the type, room temperature, humidity levels, and surface type. For instance, water-based polyurethane will cure within 24 hours, that’s because it uses water as its solvent. While oil-based polyurethane will take 48 hours to fully harden because it uses oil as its solvent.
When the sealant has fully cured and hardened, then it can be sanded. Polyurethane is a thin finish so if you sand it before it has completely cured, you will risk damaging the coat.
Related Read: Do You Need to Sand Between Coats of Polycrylic?
Can You Skip Sanding?
You shouldn’t apply polyurethane without sanding the surface first. This is because polyurethane is a clear finish. So, if you don’t sand, the imperfections will show after the sealant dries.
You should also sand before applying it to get rid of any blemishes, pencil marks, glue, or bumps on the surface. All of which can affect the application and curing process of the finish.
Bare wood especially should be properly sanded first with medium-grit sandpaper. And, then fine-grit sandpaper before applying the sealant. Wooden surfaces usually have pencil marks, glue, and even dents left behind by the carpenter when the furniture was made. All of these prevent good adhesion and a smooth finish.
To sand bare, you should start with medium-grit sandpaper (120/150-grit). Then you should move to finer-grit sandpaper to smoothen the surface. To sand painted surfaces, use fine-grit sandpaper. This sandpaper will remove the imperfections over the painted surface, but not the paint.
Related Read: Can You Apply Polyurethane Over Old Polyurethane?
Sanding Between Coats
Sanding polyurethane is different from regular sanding. When you sand the sealant, you are not trying to strip the finish, you just want to remove imperfections and abrade the previous coat. So, you shouldn’t remove the glossy layer or the entire finish, only smooth it.
To sand polyurethane, use fine-grit sandpaper as the sealant isn’t that thick. If you use medium or coarse-grit sandpaper you can remove the entire finish.
To sand it, swipe the sandpaper over the coating a few times or until the imperfections are gone. If the previous coating doesn’t have many imperfections, use very fine-grit sandpaper (400-grit) and don’t sand as much.
You shouldn’t sand the final coating of the polyurethane. If you do, you will remove its glossy and moisture-resistant layer and expose the surface underneath to weather elements.
The main purpose of a sealant is to protect the finish underneath it. They protect a surface by producing a glossy and moisture-resistant layer that prevents liquid from penetrating its coating and reaching the finish underneath. So, if you sand the final coating and remove the glossy layer, the finish underneath gets exposed.
However, if the final coating has imperfections, you can lightly sand it using very fine-grit sandpaper. This sandpaper won’t remove the glossy layer, but instead, it will remove imperfections and bumps over it.
Related Read: Should You Sand Coats of Stain?
If You Don’t Sand:
If you don’t sand between coats of polyurethane, the finish won’t come smooth. The dust trapped on the existing coat won’t allow the new coating to stick properly. Due to the dust, the new coating can also turn sticky.
Since polyurethane is a thin finish, brush marks can easily appear on the coat. If you don’t sand that coat after it has dried, the brush marks will show over the next coat. Brush marks won’t disappear until you sand the coating.
Also, polyurethane has a glossy moisture-resistant finish that doesn’t allow moisture to penetrate it. So, if you don’t sand between coats, the glossy moisture-resistant layer won’t allow the new coating to stick.
You should always sand between coats of polyurethane to get a smooth and even coating. If you don’t, the new coating won’t stick properly and the finish will be riddled with bumps, dust, and imperfections.