Can You Apply Varnish Over Polyurethane? (& Vice Versa)

Wondering if you can apply varnish over polyurethane? Well, that makes you and many other DIYers. So can you?

You shouldn’t apply varnish over polyurethane. This is because polyurethane is a moisture-resistant film while varnish is a penetrating finish. This means that polyurethane repels water and oil.

Seeing as varnish is an oil finish, it won’t penetrate the polyurethane. Since varnish needs to penetrate to stick, it won’t stick on polyurethane and will peel off after a few days.

The only possible way to varnish over polyurethane is to sand the polyurethane finish first to create pores in the finish. The varnish when applied will penetrate the pores and stick better. You should know that water-based varnish will perform better on polyurethane than oil-based varnish since water-based polyurethane is thinner.

Let’s go deeper into this topic.

Is Varnish Compatible With Polyurethane?

Varnish is compatible with oil-based polyurethane. But, the proper way to use varnish and polyurethane is to apply the varnish first and the polyurethane after. Since polyurethane is a sealant, it doesn’t need to penetrate the wood to stick or dry properly like varnish. This means that polyurethane will stay on the varnish easily.

You shouldn’t use varnish over polyurethane unless there is no other option. This is because varnish is not compatible when used over polyurethane.

Polyurethane when dry forms a very hard finish that oil or water can’t penetrate or soak in. Varnish by design is in oil form and the finish has to penetrate to stick and dry properly. Since the varnish can’t penetrate the hard polyurethane finish, it won’t stick.

If you are to use varnish over polyurethane because it’s the last option, you have to sand the polyurethane finish to create pores and spaces in the finish that the varnish can penetrate through. Let’s check out how to varnish over polyurethane in detail.

Related Read: Polyurethane vs Varnish?

How To Apply Varnish Over Polyurethane?

To varnish over polyurethane, you’ll need a few tools and supplies:

  • Sandpaper
  • Rags
  • Varnish (preferably water-based varnish)
  • A bristled paintbrush
  • A pair of gloves
  • Paint Thinner
  • Degreaser
  • A face mask

Here is a 5-step guide that discusses how to varnish over polyurethane:

1. Wipe and Degrease The Polyurethane

Wipe and Degrease The Polyurethane

The first step is to wipe the polyurethane. This step removes dust nibs and grain on the polyurethane that can gunk up the sandpaper. Also, if the polyurethane is filthy, it will be very difficult for the varnish to stick.

You should wipe the polyurethane with a clean rag. You should also use a degreaser to remove oils and grease on the polyurethane as these can cause stains and bleed through.

2. Sand The Polyurethane

Sand The Polyurethane

When the polyurethane is clean, you should sand it with medium-grit sandpaper. 150-grit sandpaper works fine. The sandpaper is not very fine and not very coarse so it’s perfect for creating pores and scars in the polyurethane that the varnish can soak into.

After sanding with medium-grit sandpaper, you should finish off with fine-grit sandpaper. While medium-grit sandpaper helps to create pores in the polyurethane, fine-grit sandpaper helps to smoothen the surface so the polyurethane can go on smoothly.

Ensure to put on breathing protection and a pair of gloves as sanding polyurethane can spew fumes in the air.

After sanding, you are sure to have dust and grain on the polyurethane. You need to wipe off the dust if not, there will be pimples and bumps in the varnish. You can wipe the polyurethane with a clean cloth.

3. Thin The Varnish

Thin The Varnish

Varnish in its original state is very thick. If you thin the varnish, it will be easier to apply and control. To thin water-based varnish, you can use ½ cup of water. To thin oil-based varnish, you can use mineral spirits or lacquer thinner.

Pour the varnish first into a clean bucket. Then measure the right amount of lacquer or paint thinner and add it to the varnish. When this is done, you’ll have to stir the mixture with a turning stick or a paint mixer.

Don’t stir too hard or you’ll cause bubbles in the varnish. When you are done stirring, test the varnish on a part of the wood to see how well it goes on and how it dries. If the varnish dries well, you can put it on the remaining wood.

4. Apply The Varnish

Apply The Varnish

It’s better to apply varnish with a bristled paintbrush as it allows the varnish to get into the wood grain. Also, using a paintbrush gives you better control over the varnish and you can easily apply thin coats.

You can apply up to 3 coats of varnish but seeing as there is a polyurethane layer underneath, you only need 2 coats for proper coverage.

Ensure to allow each coat of varnish to dry for about 4 hours before a recoat. If you didn’t thin the varnish, you have to wait at least 6 hours before a recoat. It’s advised to sand between coats of varnish. This is to remove dust nibs that settled on the existing coat during its 4-6 hours dry time.

You shouldn’t sand the final coat of varnish as the final coat is meant to reveal the hard glossy finish. Ensure to leave the varnish for at least 3 days after application before using it or placing objects on it.

If you applied the varnish over a polyurethane-coated floor, you should let it dry fully before walking on it. This can take a few days or weeks depending on the drying conditions.

Can You Varnish Over Polyurethane Without Sanding?

You shouldn’t varnish over polyurethane without sanding. If you apply varnish over polyurethane without sanding, the varnish will not stick well and will peel off in a few days. This is because the varnish will not penetrate the polyurethane coating.

As explained earlier, the purpose of sanding polyurethane before the varnish is to create pores in the polyurethane to allow the varnish to penetrate the polyurethane coating. If you don’t sand, there wouldn’t be any space for the varnish to penetrate through meaning that it will not stick.

If you don’t sand polyurethane before varnishing,

  • The varnish will not stick properly
  • The varnish will not dry properly
  • The varnish will be prone to scratches, moisture, and heat
  • The varnish will turn tacky
  • The varnish will not be strong or durable
  • The varnish will peel off eventually.

So sanding is important. But can you mix varnish with polyurethane? Let’s take a closer look.

Can You Mix Varnish With Polyurethane?

You can mix varnish with polyurethane but the varnish and polyurethane have to be the same base. This means that you can only mix water-based varnish with water-based polyurethane. And, oil-based varnish only with oil-based polyurethane.

Since oil and water aren’t compatible, mixing polyurethane and varnish of different bases will result in a finish that you can’t use. Asides from that, the finish will not dry properly and there will be massive color variations in the finish.

You should also know that mixing polyurethane and varnish will produce a finish that will be very thick making its application difficult. If you mix varnish and polyurethane, you’ll have to thin the mixture heavily so it can be easier to apply and so it can dry faster.

Can You Apply Polyurethane Over Varnish?

You can put polyurethane over varnish. This is because polyurethane is a sealant and by design, it’s meant to be put over something.

Being a sealant, polyurethane doesn’t need to penetrate to stick. Since it doesn’t need to penetrate, polyurethane will stick easily to the top layer of the varnish.

You should know that you have to sand the varnish to allow the polyurethane to dry and come out smooth. Sanding also helps to remove imperfections in the varnish that can repel polyurethane. You should sand with fine-grit sandpaper.

Final Words

Overall, it’s not advised to put varnish over polyurethane because varnish is a penetrating oil finish and polyurethane will not let the varnish penetrate or stick.

The only way to put varnish over polyurethane is to sand the polyurethane properly before applying the varnish. This way, the varnish can bite into the polyurethane to stick better.

On the other hand, polyurethane will stick better to varnish since it’s a sealant and doesn’t need to penetrate.

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