Polyurethane Over Linseed Oil: (Can You Do it & Benefits)

Polyurethane protects wood from scratches, while linseed oil seals the wood grain. So, can you apply polyurethane over linseed oil?

You can apply polyurethane over linseed oil. Polyurethane doesn’t need to penetrate a surface to stick, so it will stick over linseed oil. But, the linseed oil must be fully dry. You can use both; water-based and oil-based polyurethane over boiled linseed oil. 

On the other hand, you can’t apply linseed oil over polyurethane. That’s because linseed oil is a penetrating finish (it needs to penetrate the surface to stick), while polyurethane prevents liquid from penetrating its surface. 

Polyurethane Sticks To Linseed Oil

Polyurethane sticks to linseed oil. But, the linseed oil must be fully dry, and you must sand the wood oil before applying polyurethane.

If you apply polyurethane before linseed oil dries, the polyurethane and linseed oil will mix and create a rich oil varnish that won’t harden or cure. This will ruin and finish. 

Also, you must sand linseed oil with ultra fine-grit sandpaper before applying polyurethane. Sanding will remove the dirt and imperfections from the linseed oil, allowing polyurethane to stick to linseed oil better. 

To improve the bonding between oil-based polyurethane and linseed oil, thin the oil-based polyurethane with mineral spirits before applying it. The lighter the flow of polyurethane is, the better it sticks and the faster it dries. 

Oil-based polyurethane sticks to linseed oil better than water-based polyurethane. That’s because water-based polyurethane uses water as its solvent and might not stick well over the linseed oil.

To apply water-based polyurethane over linseed oil, you must apply a thin coat of water-based primer first and then apply the polyurethane. The oil-based primer must be compatible with water-based polyurethane.

Also, 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, you must wait several weeks for the raw linseed oil to dry before applying polyurethane. 

How To Apply Polyurethane Over Linseed Oil?

You must only apply polyurethane once the linseed oil is fully dry. 

Here are the tools you need:

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

1. Clean and Sand The Linseed Oil

Clean and Sand The Linseed Oil

First, cover the floor with a drop sheet to prevent paint spills. Move the furniture and objects away from the surface to be painted. Also, wear safety goggles. 

Next, clean and sand the linseed oil. 

To clean linseed oil:

  1. Wait for the linseed oil to dry.
  2. Damp a clean rag with soapy water. 
  3. Wipe the dampened rag over the linseed oil coating. 
  4. Wait 2 minutes.
  5. Remove the soapy water from the linseed oil coating.
  6. Allow the surface to dry. 

Once the linseed oil is clean, sand it. Sanding will remove small imperfections from the surface. To sand linseed oil, use 320-grit sandpaper. Don’t use medium or coarse-grit sandpaper as it will damage the linseed oil. After sanding, remove the dust using a clean rag. 

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

Apply Paint Primer

If you want to use water-based polyurethane over linseed oil, apply an oil-based primer first. You don’t need to apply a primer if you use oil-based polyurethane. You can apply the oil-based polyurethane directly over the linseed oil.

The oil-based primer will produce a thin coating over linseed oil, allowing the water-based polyurethane to stick. The oil-based primer must be compatible with the water-based poly. However, the primer will prevent the color shade of linseed oil from showing on the finish, so the finish will have the same color as the primer.

Apply one thin coat of oil-based primer and allow it to dry. 

3. Thin The Polyurethane

Thin The Polyurethane

Polyurethane and linseed oil both have a thick flow. If you apply un-thinned polyurethane over linseed oil, the finish will be thick. So, to get a thinner finish, you must thin the polyurethane first. 

To thin oil-based polyurethane, use mineral spirits. To thin water-based polyurethane, use water. Thin the polyurethane with a ratio of 2:1 (2 parts polyurethane, 1 part paint thinner). 

4. Apply The Polyurethane

Apply The Polyurethane

To apply polyurethane over linseed oil:

  1. Use a paintbrush, sprayer, or roller. 
  2. Apply 3 thin coats of polyurethane. 
  3. Wait until one coat dries before applying the next one.
  4. Once you apply the final coat, wait until the finish fully dries (cures) before using it. 

Mixing Linseed Oil and Polyurethane

You shouldn’t mix linseed oil and polyurethane. That’s because they are different types of topcoats and have different additives. A mixture of polyurethane and linseed oil will produce a thick coat that is hard to apply. 

Also, linseed oil and polyurethane have different drying times. So, one part of the mixture will dry while the other will remain wet for several days. This leads to a cracked finish. Polyurethane dries within 24 hours, while linseed oil requires several days to dry.

Linseed oil is a penetrating topcoat, while polyurethane is a tropical topcoat (it doesn’t need to penetrate to stick). So, the mixture won’t stick properly to surfaces. 

Also, mixing polyurethane and linseed oil will give you a mixture with a yellow tint due to the high volume of oils in the mixture. So, if you apply the mixture to wood, the wood will have an amber-tone finish that won’t dry. 

You shouldn’t mix polyurethane with linseed oil. You should only apply either on the other.

Linseed Oil Over Polyurethane

Linseed oil won’t stick to polyurethane unless you remove the polyurethane first. That’s because linseed oil is a penetrating finish, and polyurethane coating prevents liquid from penetrating its surface.

So, if you apply linseed oil over polyurethane, the linseed oil won’t be able to penetrate the polyurethane coating and won’t stick. This leads to sticky or tacky linseed oil.

So, to apply linseed oil over polyurethane, you must sand or remove the polyurethane first. Sanding polyurethane will create tiny ridges (holes) that the linseed oil can penetrate into. This allows linseed oil to stick, but the bonding between linseed oil and polyurethane won’t be good.

So, the best way to apply linseed oil over polyurethane is to remove the polyurethane top layer. With the top layer removed, the linseed oil can stick.

Final Words

You can apply polyurethane over linseed oil because polyurethane is a tropical topcoat that doesn’t need to penetrate to stick. However, the linseed oil must be cleaned and sanded before applying polyurethane. 

On the other hand, you can’t apply linseed oil over polyurethane. That’s because linseed oil is a penetrating finish, and polyurethane prevents liquid from penetrating its surface. So, linseed oil can’t stick over polyurethane. You must sand or remove the top layer of polyurethane for the linseed oil to stick. 

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