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

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. 

Compatibility

Polyurethane sticks to linseed oil but the coating must be fully dry, and you must sand the wood oil before sealing it. If you seal it too soon, both finishes will mix and create a rich oil varnish that won’t harden or cure. 

You must sand linseed oil with ultra fine-grit sandpaper before sealing it. Sanding will remove the dirt and imperfections from the finish, allowing the sealant to stick better. 

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

Oil-based polyurethane sticks to linseed oil better than water-based polyurethane because it uses oil as its solvent and is compatible with linseed oil. 

To apply water-based polyurethane over it, you must apply a thin coat of oil-based primer first. The primer must be with water-based polyurethane for it to stick. 

You can also only apply it over boiled linseed oil since it dries faster than raw linseed oil. If you apply it over raw linseed oil, you must wait several weeks for the raw linseed oil to dry.

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

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 finish

To clean it:

1. Wait for the linseed oil to dry.
2. Damp a clean rag with soapy water. 
3. Wipe the dampened rag over the coating. 
4. Wait 2 minutes.
5. Remove the soapy water from the finish.
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 finish. After sanding, remove the dust using a clean rag. 

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

If you want to use water-based polyurethane, apply an oil-based primer first. The oil-based primer will produce a thin coating over the finish, allowing the water-based polyurethane to stick. The 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

If you apply un-thinned polyurethane, the finish will be too thick. So, to get a thinner finish, you must thin it.

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

4. Apply The Polyurethane

To do it:

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 them will produce a thick coat that is hard to apply. 

Also, they 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 them 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 the wood, the wood will have an amber-tone finish that won’t dry. 

In summary, you shouldn’t mix these two finishes, you can only apply one over the other.

Linseed Oil Over Polyurethane

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

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

You must sand the polyurethane first to create tiny ridges (holes) that the linseed oil can penetrate into. This allows it to stick, but the bonding between them won’t be good.

So, the best way to apply it is to remove the polyurethane top layer first.

Final Words

You can apply polyurethane to 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.

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.