Can You Put Polyurethane Over Linseed Oil? (or Vice Versa)

Polyurethane is known to protect wood from scratches while linseed oil is known to seal wood grain. But can you put polyurethane over linseed oil?

Yes, you can put polyurethane over linseed oil. But, you need to ensure the linseed oil is completely dry before polyurethane application. Both water-based and oil-based polyurethane can be used over linseed oil. But, you can only apply polyurethane over boiled linseed oil.

You can also put linseed oil over polyurethane too but you can only do this with oil-based polyurethane. Also, you’ll need to strip or sand the oil-based polyurethane to allow the linseed oil to stick.

Will Polyurethane Stick To Linseed Oil?

Polyurethane will stick to linseed oil only if the linseed oil is completely dry and you sand it before applying the polyurethane.

It’s important to allow the linseed oil to dry before you apply polyurethane over it. If the linseed oil isn’t fully dry, then the polyurethane coating will mix with the linseed oil and create a rich oil varnish that will not harden or cure.

It’s also important to sand the linseed oil with an ultra fine-grit sandpaper too. This is to remove dirt on the linseed oil and to allow the polyurethane to bite into the linseed oil. This helps the polyurethane to stick better.

You should also know that oil-based polyurethane sticks better to linseed oil than water-based polyurethane. This is because water-based polyurethane is dissolved in water and might not stick well over linseed oil.

To apply water-based polyurethane over linseed oil, it’s advised to apply a thin coat of oil-based primer on the linseed oil first. This is to make the water-based polyurethane stick better. You should ensure to use an oil-based primer that is also compatible with water-based polyurethane.

You can also make an oil-based polyurethane stick better to linseed oil by thinning the oil-based polyurethane with mineral spirits before applying it. When you thin the polyurethane, it becomes lighter and this makes it stick better and dry faster on the linseed oil.

You can only apply polyurethane over boiled linseed oil since boiled linseed oil dries faster than raw linseed oil. If you apply polyurethane over raw linseed oil, the polyurethane will most likely not stick well because raw linseed oil can take several weeks to dry.

Can You Mix Linseed Oil and Polyurethane?

It’s not a good idea to mix polyurethane with linseed oil. This is because doing so will make the mixture very thick and difficult to apply.

If you mix polyurethane and linseed oil, the mixture will be very thick. This will make it more difficult to apply the mixture. Also, the mixture will not cure or harden. This is because linseed oil and polyurethane have different drying times.

It takes polyurethane about 24 hours to dry while linseed oil requires several days to fully set. So if you mix polyurethane and linseed oil, you’ll get a mixture that has different drying times meaning that the mixture will never dry or harden completely.

You should also know that mixing polyurethane with linseed oil will give you a mixture that will have a very yellow tint due to the high volume of oils in the mixture. So if you put this mixture over wood or over a stain, the stain will look yellow or have an amber tone.

You shouldn’t mix polyurethane with linseed oil. You should only apply either on the other. Let’s find out how to apply polyurethane over linseed oil.

How To Apply Polyurethane Over Linseed Oil? (4 Steps)

This guide will show you how to apply polyurethane over linseed oil in 5 simple steps. But first, you’ll need a few tools and supplies:

  • A bristled paintbrush
  • A pair of gloves
  • Paint thinner or mineral spirits
  • Clean rags
  • Fine and ultra-fine grit sandpaper

Before we go on, you should remember that you can only apply polyurethane over boiled linseed oil. Here is a 4-step guide on how to apply polyurethane over boiled linseed oil:

1. Clean and Sand The Linseed Oil

Clean and Sand The Linseed Oil

Ensure to cover the floor with a drop sheet to prevent stains. You should also put on a pair of gloves and breathing protection (or a face mask). All furniture and objects on or around the surface to be painted should be removed or covered with tape to prevent polyurethane paint stain.

The next step is to clean and sand the linseed oil. Start by wiping the linseed oil surface with a clean rag to get rid of dust. Then sand the linseed oil with very fine sandpaper.

I’ll recommend that you use 320-grit to sand the linseed oil. Medium and coarse grit sandpaper will damage the linseed oil finish. After sanding, ensure to remove dust with a vacuum or clean rag.

2. Apply Paint Primer (For Water-Based polyurethane)

Apply Paint Primer

This step is for water-based polyurethane. If you want to apply water-based polyurethane, it’s better to apply an oil-based primer on the linseed oil. This is to allow the water-based polyurethane to stick better. Ensure to use an oil-based primer that is compatible with water-based polyurethane.

You should know that without an oil-based primer coating, the water-based polyurethane will not stick well. The reason you need to use primer is to make the paint adhesion stronger.

All you need is a coat of oil-based primer. Ensure to allow the paint primer to dry properly before moving on to the next step.

3. Thin The Polyurethane

Thin The Polyurethane

Polyurethane has a thick flow. The thickness of the paint can make it difficult to apply it over linseed oil which for the record is also thick. So it’s generally advised to thin the polyurethane before applying it over linseed oil.

You don’t have to thin water-based polyurethane since water-based polyurethane is not as thick as oil-based polyurethane.

To thin the polyurethane, you should use mineral spirits. You can mix the mineral spirits and oil-based polyurethane in the ratio 1:2 (1 part mineral Spirit to 2 part polyurethane). Ensure not to over-thin the polyurethane.

4. Apply The Polyurethane

Apply The Polyurethane

The final step is to apply the polyurethane. You should use a natural bristled paintbrush for oil-based polyurethane and a synthetic-bristled paintbrush for water-based polyurethane.

You can also spray the polyurethane. Spraying the polyurethane helps with better coverage and you can finish the paint job quicker.

After applying the polyurethane, you should wait at least 48 hours for it to fully dry.

Will Linseed Oil Stick To Polyurethane?

Linseed oil will not stick to polyurethane unless you remove the polyurethane finish from the surface. This is because linseed oil is a penetrating oil finish.

This means that for linseed oil to stick, it needs to penetrate and soak in the wood or material. Polyurethane on the other hand is a moisture-resistant finish. This means that polyurethane will not allow oil or water to penetrate it.

Since linseed oil needs to penetrate and polyurethane doesn’t allow this, linseed oil will not stick to polyurethane. You can only put linseed oil over polyurethane if you remove the polyurethane completely.

The linseed oil can also stick better if you remove the top layer of the polyurethane. With the top layer removed, the linseed oil can stick better but this only works for oil-based polyurethane.

You can remove polyurethane using a paint stripper, a paint scraper, or a paint thinner.

Can You Put Linseed Oil Over Polyurethane?

Yes, you can put linseed oil over polyurethane. But, you should know that linseed oil works best when used over oil-based polyurethane due to the high level of oils in both finishes. When you put linseed oil over polyurethane, you’ll create a finish known as “French polish.”

If you put linseed oil over water-based polyurethane, the linseed oil will not stick well because water-based polyurethane is dissolved in water. To put linseed oil over water-based polyurethane, you’ll have to remove the water-based polyurethane first so the linseed oil can penetrate.

You should know that if you want the linseed oil to work well over a polyurethane-painted surface, you have to strip or remove the polyurethane finish regardless of if the polyurethane is oil or water-based.

Final Words

Overall, you can put polyurethane over linseed oil since polyurethane doesn’t need to penetrate to stick. You just have to ensure that the linseed oil is properly cleaned and sanded to allow the polyurethane coat to work well.

To put linseed oil over polyurethane, you’ll most likely need to remove or strip the polyurethane finish. You should also remember that linseed oil works best with oil-based polyurethane.

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