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 the sealer doesn’t need to penetrate the surface to stick. But, Linseed oil must be fully dry before you apply the polyurethane.
You can’t apply linseed oil over polyurethane because linseed oil is a penetrating finish (it needs to penetrate the surface to stick), while polyurethane prevents liquid from penetrating its surface.
Does Polyurethane Adhere Over Linseed Oil?
Polyurethane does adhere over Linseed oil, but the finish must be dry and sanded 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 sealer to stick better.
Thin oil-based polyurethane with mineral spirits before applying it over Linseed oil to improve the bonding between both finishes. The thinner (lighter) the flow is, the better it will stick and the faster it will dry.
Oil-based polyurethane sticks to linseed oil better than water-based polyurethane because their solvents (oils) are compatible. You must apply a thin coat of oil-based primer if you want to use water-based polyurethane.
Polyurethane sticks to boiled Linseed oil as it dries faster than raw Linseed oil. If you want to seal raw Linseed oil, you must wait several weeks for it to fully dry.
How To Apply Polyurethane Over Linseed Oil?
To apply polyurethane over Linseed oil, do the following things.
- Clean and Sand the Linseed Oil.
- Apply Paint Primer (For Water-Based polyurethane Only).
- Thin Polyurethane.
- Apply Polyurethane.
The tools you need for this project are listed below.
- 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
Cover the floor with a drop sheet to prevent paint spills, move the furniture and objects away, and wear safety goggles. Next, clean the finish.
To clean the Linseed Oil finish, do the following things.
- Wait for the linseed oil to dry.
- Damp a clean rag with soapy water.
- Wipe the dampened rag over the coating.
- Wait 2 minutes.
- Remove the soapy water from the finish.
- Allow the surface to dry.
Sand the Linseed oil finish after you clean it. Sanding will remove small imperfections from the surface and leave behind a textured finish.
To sand Linseed oil, use fine-grit sandpaper (320-grit). Don’t use medium or coarse-grit sandpaper as it will damage (or remove) the entire finish. Clean the finish (again) with a rag after sanding it.
2. Apply Paint Primer (For Water-Based Polyurethane Only)
Apply a compatible oil-based paint primer if you want to apply water-based polyurethane over Linseed oil.
The oil-based primer will produce a thin coating over the finish, allowing the water-based polyurethane to stick.
The primer will prevent the color shade of the Linseed oil from showing over the finish — the finish will have the same color as the primer. If you don’t want that, skip this step.
3. Thin Polyurethane
Un-thinned (normal) polyurethane has a thick flow and will make the finish too thick.
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 Polyurethane
To do it:
- Use a paintbrush, sprayer, or roller.
- Apply 3 thin coats of polyurethane.
- Wait until one coat dries before applying the next one.
- Once you apply the final coat, wait until the finish fully dries (cures) before using it.
Can You Mix Linseed Oil and Polyurethane?
You can’t mix linseed oil and polyurethane as they are different types of topcoats and have different additives. A mixture of them will produce a thick coat that is hard to apply and won’t dry.
Both finishes have a different dry time — one part of the mixture will dry while the other will remain wet for several days, leading 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 topical topcoat (it doesn’t need to penetrate to stick). So, the mixture won’t stick properly to surfaces.
A mixture of polyurethane and Linseed oil will produce a yellow tint over the finish due to the high volume of oils. If you apply the mixture to a wooden surface, the surface will have an amber-tone finish that won’t dry.
Can You Apply Linseed Oil Over Polyurethane?
You can’t apply linseed oil over polyurethane as it won’t stick unless you remove the sealer first. Linseed oil is a penetrating finish, and polyurethane coating prevents liquids from penetrating its surface.
The wood oil won’t be able to penetrate the coating or stick if you apply it over polyurethane. This will lead 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 and then apply the wood oil.
