If you’re building a table, working on a vanity or applying the final touches to any other woodworking project, your thoughts are likely to be turning towards the finish you’re going to apply when it’s done.
Specifically, you might be considering an oil and polyurethane combination to bring out the wood while also protecting it from water and scratches – but is this a viable option?
To help you understand whether this works and how to do it, in this post, we answer the question, can I apply polyurethane over an oil finish?
Can I Apply Polyurethane Over an Oil Finish?
1. Oils and varnishes
When you’re coming to the end of any woodworking project, one of the last things you’ll have to do is apply the finish.
In woodworking, finishes have two equally important roles – to enhance the appearance of the wood and to protect it against water, mold, wood-eating insects and marks left by dirty hands.
Many newbies to woodworking assume that when it comes to finishes, the options are basically limited to paint or varnish, but there’s a lot more to it than this.
To provide a beautiful yet protective coat, you can also consider oils – and oils can be used in combination with varnishes to provide the wood with even more protection.
Oils can be applied to wood to bring out the natural colors, and they also protect the wood from the inside by preventing it from drying out.
Varnishes, on the other hand, can provide a protective outer layer. However, “varnish” isn’t a product in itself, and there are several options for varnishes, including, for example, lacquer.
Polyurethane is another substance that can be used as a varnish, and the theory is that when combined with an attractive oil, you can give your project a finish that is both protective and aesthetically pleasing.
The question is, then, does this work in practice?
The answer is that it does, but it depends on the oil you use – so let’s look at this now.
2. Drying oils and non-drying oils
There is a whole range of oils that can be used as a finish on wood, and broadly speaking, they can be divided into two types – drying oils and non-drying oils.
At the most basic level, drying oils are those that dry hard and cure, forming a solid film when exposed to the oxygen in the air.
Examples of drying oils are tung oil (or China wood oil, which comes from the nut of the tung tree), linseed oil and walnut oil.
Non-drying oils, on the other hand, are oils that will never fully dry or cure into a solid film, however much time they are given.
Examples of non-drying oils include olive oil, palm oil and hazelnut oil.
Since drying oils cure into a solid surface, it is possible to then apply polyurethane over the top since it has something to bind to.
However, because non-drying oils will never cure in this way, polyurethane can’t be applied over the top since there is no solid surface for it to stick to.
3. How do you know which oils you can use?
Now we understand that whether you can apply polyurethane over oil depends on whether the oil you are using is a drying or non-drying oil, your next question is likely to be, how do you know if a particular oil is drying or non-drying?
To answer that, now we need to do the science bit.
Oils are hydrocarbons, which means they consist entirely of hydrogen and carbon atoms, and they can be saturated or unsaturated
Saturated oils contain only single bonds – and as a result, we say they are saturated with hydrogen.
Unsaturated, on the other hand, also contain double or triple bonds between carbon atoms – and the more double or triple bonds there are, the more unsaturated the oil is said to be.
We can test how unsaturated and oil is by adding iodine, and the reaction to iodine gives us the so-called iodine number of an oil.
As a rule, oils with an iodine number below 115 are considered non-drying oils while those above 115 are considered drying oils.
Here’s a table of many common oils along with their iodine numbers.
|Canola oil||110 – 126|
|Castor oil||81 – 91|
|Coconut oil||6 – 11|
|Cod liver oil||148 – 183|
|Corn oil||107 – 128|
|Cottonseed oil||100 – 115|
|Fish oil||190 – 205|
|Grape seed oil||94 – 157|
|Hazelnut oil||83 – 90|
|Jojoba oil||80 – 85|
|Kapok seed oil||86 – 110|
|Linseed oil||170 – 204|
|Olive oil||75 – 94|
|Oiticica oil||139 – 185|
|Palm kernel oil||14 – 21|
|Palm oil||49 – 55|
|Peanut oil||82 – 107|
|Pecan oil||77 – 106|
|Pistachio oil||86 – 98|
|Poppyseed oil||140 – 158|
|Rapeseed oil||94 – 120|
|Rice bran oil||99 – 108|
|Safflower oil||135 – 150|
|Sesame oil||100 – 120|
|Sunflower oil||110 – 145|
|Soybean oil||120 – 139|
|Tung oil||160 – 175|
|Walnut oil||132 – 162|
|Wheat germ oil||115 – 128|
How to Apply Polyurethane Over an Oil Finish?
So now we know about which oils can and can’t be used with polyurethane, how should you go about applying such a finish? Here’s how to do it.
Step 1. Choose your oil and apply it to the wood
The first step is to choose the oil you want to use – and when choosing, you need to make sure it is a drying oil. Then, once you have chosen your oil, you need to apply it to the wood in an even coat.
Once applied, the oil will seep into the wood, and any excess should be wiped off.
Step 2. Leave the oil to dry and cure
After applying your oil to the wood, you need to let the oil dry and cure properly before applying the polyurethane.
The amount of time it takes for this to happen will depend on factors such as the temperature, humidity and wind conditions where you live, but it will take at least three days – and to be safe, if you can wait, it’s best to give it as long as a week.
Step 3. Sand the surface
When the oil has cured, you then need to give the surface a light sanding.
This is because one of the main problems people discover when trying to apply polyurethane over oil is that the polyurethane has trouble adhering to the surface.
Sanding the surface will resolve this issue since it will give the polyurethane a slightly rough surface to fix itself too.
However, you don’t want to sand it too vigorously or too deeply or it will spoil the appearance of the wood.
For best results, you should use very fine grit sandpaper – around 220-grit or 320-grit will be about right.
Step 4. Apply a thin coat of polyurethane
Next, you should apply a thin coat of polyurethane. To achieve this, it’s best to thin the polyurethane before applying it. This can be achieved by mixing two parts polyurethane with one part mineral spirit.
Alternatively, you can buy a polyurethane product that is pre-thinned.
When the polyurethane is ready, apply it evenly to the wood with a brush and leave it to dry.
Step 5. Sand the polyurethane and apply further coats
Once the polyurethane layer is dry, you can then apply a second and even a third coat.
The important thing to remember is that you should sand each coat to provide a slightly rough surface for the next coat to adhere to.
Step 6. Sand the final layer
Since polyurethane isn’t self-levelling, you’ll also need to sand it lightly once the final coat has dried. This will help give you a smooth, flat surface.
And once this is finished, the job is done.
What about Danish oil or Tru-Oil?
What about other products such as Danish oil or Tru-Oil? Can you apply a polyurethane coat over these?
The thing to understand about products like these is that they are not just oil but rather mixtures of oil and varnish that can be used alone to finish a wood surface.
When it comes to Danish oil, there is no set recipe, and each manufacturer creates its own blend.
However, this usually consists of two parts varnish and one part oil, and with a formula like this, if it is left to dry, it will be able to hold a coat of polyurethane just the same as any drying oil would.
Tru-Oil is a similar product that contains oil and varnish, and just like Danish oil, it will be able to hold a coat of polyurethane.
Adding one or more coats of polyurethane to finishes like Danish oil or Tru-Oil will give the final finish an extra layer of protection – just make sure you sand each layer as you apply them, as explained above.
Polyurethane works with drying oils
As we’ve seen, polyurethane can be combined with oils, as long as the oil in question is a drying oil.
The key to success is to leave the oil to dry and cure so it presents a hard surface for the polyurethane to bind to – and an important tip is to sand each layer before you apply the next to help each layer adhere more firmly to the layer below.