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 polyurethane finish?
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 polyurethane while it’s drying.
However, this is just a snippet. There is more to know about polyurethane finish and this post digs right into the topic. Let’s dive in.
Why Should You Sand Between Coats of Polyurethane?
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 polyurethane after it has dried. If these imperfections are left on the polyurethane surface, the next coats of polyurethane won’t stick properly. The overall finish will also come out rough, uneven, and ugly.
To get a smooth and fine polyurethane 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 polyurethane coating. So, if you don’t sand the polyurethane 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.
So it’s safe to say that if you apply multiple coats of polyurethane without sanding between coats, you won’t get a smooth and fine result.
However, you can apply polyurethane without sanding if you apply perfect coats and the surface isn’t filled with dust. But, that’s very unlikely. So, always sand between coats of polyurethane.
How Long To Wait Before Sanding Polyurethane?
You have to wait until the polyurethane cures before sanding it. It takes between 12 – 48 hours for polyurethane to fully cure and harden. On average, you have to wait 24 hours after applying polyurethane before you can sand the finish.
Sanding between coats of polyurethane is advised. But, you need to wait for the polyurethane finish to completely cure and harden before you sand it. This is why it takes a while to complete the application of the polyurethane finish.
The time it takes for the polyurethane to fully cure and harden depends on the type of polyurethane you applied. Water-based polyurethane for instance will cure within 24 hours, that’s because it uses water as its solvent.
Oil-based polyurethane can take up to 48 hours to fully harden. This is because oil-based polyurethane uses oils as the solvent. And it takes longer for oil to be completely evaporated from the paint particles.
When the polyurethane has fully cured and hardened, then it can be sanded. Polyurethane is a thin finish so if you sand the coat of polyurethane before it has completely cured, you will risk damaging the coat.
The curing process is complete when the solvent is completely evaporated from the paint particles.
Related Read: Do You Need to Sand Between Coats of Polycrylic?
Can You Apply Polyurethane Without Sanding?
You shouldn’t apply polyurethane without sanding the surface first. This is because polyurethane is a clear finish. And, if you don’t sand imperfections on the surface will show after the polyurethane has dried.
You should also sand before applying polyurethane to get rid of any blemish, pencil marks, glue, or bumps on the surface. All of which can affect the application and curing process of the polyurethane finish.
When you want to apply polyurethane over any surface, it’s always a good idea to sand the surface first. Either it’s a bare surface or a stained surface, the polyurethane wouldn’t come out fine and smooth if you don’t sand the surface first.
Bare wood especially should be properly sanded first with medium-grit sandpaper. And, then fine-grit sandpaper before applying polyurethane. Wooden surfaces usually have pencil marks, glue, and even dents left behind by the carpenter when the furniture was made.
To sand the surface to be painted, you should start with medium-grit sandpaper like the 120 or 150-grit sandpaper. Then you should move to finer-grit sandpaper to smoothen the surface. After sanding, use a shop vac or vacuum cleaner to get rid of the dust and then apply the polyurethane finish.
So now you know how to sand before applying polyurethane but how do you sand coats of polyurethane finish? Keep reading to find out.
Related Read: Can You Apply Polyurethane Over Old Polyurethane?
How to Sand between Coats of Polyurethane?
Sanding polyurethane is very different from regular sanding. When you sand polyurethane, you are not trying to strip the finish. You just want to get rid of imperfections and abrade the previous coat of polyurethane. Which helps the next coat of polyurethane to bond properly and give a smooth finish.
You need fine-grit sandpaper to sand polyurethane. Poly is not a very thick finish so using medium-grit or coarse sandpaper on the polyurethane will ruin the finish. The best sandpaper to use on polyurethane starts from 240-grit and above.
To sand polyurethane, swipe the sandpaper over the polyurethane coating a few times. Sand until you notice that the bumps, grains, and dust nibs on the polyurethane have been cleared.
If the previous coat of polyurethane has dried and you can’t notice any grain or feel any bump on the surface, you don’t need a lot of sanding. Just one swipe or two of very fine-grit sandpaper like 400-grit is enough to guarantee a clear and smooth finish.
- Related Read: Do You Need To Sand Before Applying Water-Based Paint?
Should You Sand The Final Coat Of Polyurethane?
You don’t need to sand the final coat of polyurethane finish. Sanding the last coat of polyurethane finish will leave you with a scuffed and cloudy finish. However, if the last coat is looking very rough due to bad prep work and polyurethane application, you can sand lightly with 600-grit sandpaper to smoothen the finish.
Sanding polyurethane is required but only between coats of polyurethane. The final coat or topcoat of polyurethane finish doesn’t need to be sanded because you aren’t planning on applying an additional coat.
When you sand polyurethane, you remove a bit of the polyurethane layer and make it slightly rough so the next coat can stick better. But, since it’s the final coat of polyurethane, you don’t have to sand it.
The final coat should be left to dry after being applied. If needed, a sealant can be applied on the final coat but sanding isn’t required.
Related Read: Should You Sand Coats of Stain?
What Happens If You Don’t Sand Between Coats of Polyurethane?
If you don’t sand between coats of polyurethane, the finish won’t come smooth. The dust trapped between coats won’t let polyurethane stick properly. The polyurethane can also turn sticky and peel off after a while.
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 disspaar until you sand the polyurethane coat.
The imperfections the surface has will also show on the polyurethane coat if you don’t sand. If you are applying polyurethane over bare wood, all the imperfections the wood has will show after the polyurethane has dried. You should sand the bare wood to get rid of the imperfections and make the surface as smooth as possible for polyurethane.
Always sand between coats of polyurethane, especially if the previous coat isn’t smooth and fine enough. Sanding with fine-grit sandpaper will ensure that the poly cures properly and comes out fine and smooth.