What Happens if You Don’t Sand Between Coats of Polyurethane?

When applying polyurethane on a surface, painters always advise to sand between coats of the finish. But you may be wondering – what happens if you don’t sand between coats of polyurethane finish? Here is the answer to that.

If you don’t sand between coats of polyurethane, the successive coats that you apply wouldn’t bond properly to the base 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.

Can You Apply Multiple Coats Of Polyurethane Without Sanding?

What happens if you add too many polyurethane coats without sanding? You won't get a clear finish, and the paint won't stick as good as it should.

You can apply multiple coats of polyurethane without sanding as long as the surface to be painted is super smooth. And, you apply the finish in a dust-free environment.

The reason for sanding between coats of polyurethane is to get rid of dust nibs, grains, and brush marks. These things usually appear on the polyurethane after it has dried. If these imperfections are left on the polyurethane, adhesion will be difficult. The overall finish will also come out rough, uneven, and ugly.

For you to have a smooth and fine polyurethane finish, the surface and previous coats have to be smooth and fine.

When you apply polyurethane, any mistake with the brush stroke will be noticed when the paint dries because of the thin nature of the finish. As the paint dries too, dust nibs will settle on it especially if you paint in a dusty environment.

Eventually, when the polyurethane dries, you will notice the dust nibs, grains, and brush strokes on the polyurethane.

So, if you leave the coat without sanding it, the next coat you apply wouldn’t be applied on a smooth finish. Remember, polyurethane is very thin and needs to be applied on a smooth surface for the polyurethane to come out fine and smooth.

So it’s safe to say that if you apply multiple coats of polyurethane without sanding between coats, you will most likely not get a smooth and fine result.

However, you can apply multiple coats without sanding if you are perfect with your application and you paint in a super clean and dust-free environment. As long as you don’t notice any grain, dust nib, or brush mark on the polyurethane when it dries, you don’t have to sand before applying another coat.

Related Read: How To Sand Latex Paint?

How Long To Wait Before Sanding Polyurethane?

How long do you need to wait before you can sand the polyurethane coat? It depends on the polyurethane type. If it's water-based then you can sand it after 24 hours. If it's oil-based then you can sand it after 48 hours.

You have to wait until the polyurethane cures before sanding it. It takes between 12 and 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 on the other hand 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.

Polyurethane is a clear finish so if you have anything but clear, you must have sanded the finish while it was still curing. The curing process is complete when the solvent is completely evaporated from the paint particles.

Another thing to know is if you can apply polyurethane without sanding. You know you should sand between coats but should you sand before the first coat? Let’s take a closer look.

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, whatever imperfection is on the surface before applying the polyurethane, will still be obvious 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.

How To Sand Polyurethane (Made Easy)

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.

That said, 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 the coat of polyurethane, you just need to swipe the sandpaper over the polyurethane coating a few times. You do that till you notice that the bumps, grains, and dust nibs on the polyurethane have been cleared.

That said, 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.

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 are removing a bit of the polyurethane coating. You are also making the coating slightly rough so the next coat can adhere firmly. Since it’s the final coat of polyurethane, there is no reason to scuff or sand the coating because.

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: Can You Apply Polyurethane To Concrete?

Final Words

Overall, you should always sand between coats of polyurethane especially if you notice that 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.

So there you have it. If you liked this post, ensure to check out others like it on this website for more painting tips. Have a nice day.

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