Polyurethane is a common sealant used on wood because of its durability and water resistance. Shellac as well is a waxed finish commonly used on wood due to its smoothness and color. But can you apply polyurethane sealant on shellac? Here is the answer to that.
Polyurethane can be applied over shellac to give a clear finish but only if the shellac wasn’t waxed or has been dewaxed. A coat or two of Polyurethane over shellac will protect the shellac finish from moisture and heat.
On the other hand, shellac shouldn’t be applied over polyurethane. This is because the shellac finish will be too rigid to stay over flexible polyurethane. If used, the shellac will eventually split open and fall off.
But that’s not all. There is more to know about using polyurethane and shellac together. So let’s dig in.
Will Polyurethane Adhere to Shellac?
Polyurethane will adhere to shellac very well as long as the shellac finish wasn’t waxed. If polyurethane is applied over waxed shellac, the polyurethane will not stick well.
This is because the wax on the shellac will prevent the polyurethane from adhering properly. A common fix to this problem is to dewax the shellac finish since polyurethane will adhere to dewaxed shellac.
Ordinary shellac or unwaxed shellac will allow sealants like polyurethane to stick to it usually after a very light sanding. This is because the shellac wasn’t sealed and it doesn’t have any top coat that can prevent paint from bonding to it. Also, shellac is gotten from natural secretion, so there aren’t any chemicals in the shellac that can repel the polyurethane coating.
The only condition to allow polyurethane to stick to ordinary shellac is to sand. The reason for sanding is to buff the shellac finish to allow the polyurethane to sit properly.
Waxed shellac on the other hand will not let Polyurethane stick. This is because the wax on the shellac will prevent the polyurethane from adhering to the shellac underneath.
Shellac finish is not very durable and is especially prone to heat and moisture. To improve the durability of the shellac finish, wax is applied over it. The wax when dry forms a thick film that makes the shellac finish more resistant to damage. However, the wax also prevents polyurethane from adhering to the shellac.
Since the waxed shellac is hard and water-resistant, it’s usually impossible to paint over it with polyurethane unless you dewax the shellac first. This means removing the wax layer on the shellac.
Tip: You can dewax shellac by sanding or using paint-removing chemicals.
How Long Should Shellac Dry Before Applying Polyurethane?
You should let the shellac dry for 72 hours (3 days) before you seal it with polyurethane. The shellac paint has to cure before it can be sealed with polyurethane.
This means the paint solvent has to completely evaporate from the paint particles before polyurethane is applied. It takes shellac between 24 hours and 72 hours for the paint to cure enough for polyurethane top coat.
Usually, shellac is a quick-drying paint. The paint dries enough to touch under 30 minutes and you can safely recoat in about 4 hours. But for the paint to be sealed with polyurethane, you have to allow a minimum of 24 hours. 72 hours is the advised time to wait before applying polyurethane over shellac.
The reason for waiting this long before Polyurethane is applied is to give the shellac enough time to cure. Polyurethane is a plastic finish. This means that if polyurethane is applied over the shellac, nothing goes through the polyurethane, especially not solvent.
So if you don’t allow the shellac to cure properly and you rush to apply polyurethane over it, the finish will come off. This is because there will still be solvent or moisture inside the shellac coating. This moisture will not be evaporated since the polyurethane has been put over the shellac.
So instead, the moisture remains trapped underneath the polyurethane. The trapped solvent will prevent the polyurethane from adhering properly.
Also, the finish will remain tacky or sticky and will not dry for several days. When the finish eventually dries, it would have become blurry or cloudy.
Now, let’s check out how to apply polyurethane over shellac.
How to Apply Polyurethane Over Shellac (Made Easy)
Here is a quick rundown on how to apply polyurethane over shellac:
1. Cover The Floors and Walls around The Item to be Painted.
This step is very important if you plan on spraying the polyurethane on the shellac. Covering the floors and walls helps to prevent paint stains.
2. Dewax The Shellac Finish.
This step is for shellac finishes that have been waxed. You can remove the wax by sanding or using paint removing chemicals. For ordinary shellac finishes, skip this step.
3. Sand The Shellac Finish with Fine-Grit Sandpaper.
Use sandpaper from 220 grit and above to buff the shellac paint. Then wipe or vacuum the dust.
4. Apply The First Two Coats of Polyurethane.
You can apply the polyurethane by spraying or brushing it on the shellac. You should sand between coats of polyurethane.
5. Apply The Final Coat of Polyurethane
After the first two coats have dried and been sanded, you can apply the final coat. You don’t need to sand the final coat of polyurethane.
Tip: Leave the polyurethane to dry for at least 3 days before using the surface.
Can You Put Polyurethane Over Dewaxed Shellac?
You can put polyurethane over dewaxed shellac. This is because the thick layer of wax on the shellac finish has been removed.
Ordinarily, the wax on the shellac paint will prevent polyurethane from adhering to the shellac. But since the shellac has been dewaxed, the polyurethane will adhere well as long as you sand lightly to buff the shellac and remove leftover wax.
When the wax on the shellac finish is removed or stripped off, the shellac finish underneath will be exposed. Polyurethane can be used over the exposed shellac finish to protect the finish.
Shellac as explained earlier is a warm finish but not very durable. So it’s a common practice to wax over shellac to improve its durability. But many times, homeowners want to change the wax to a clear finish. To do this, you can apply polyurethane over the shellac but you have to dewax it first. So the polyurethane can stick.
To apply polyurethane over dewaxed shellac, however, you’ll need to sand the shellac with a fine-grit sandpaper. The purpose of sanding is to remove leftover wax and create tiny scratches in the shellac that the polyurethane can bite into.
Related Read: Can You Apply Polyurethane Over Gel Stain?
Can You Mix Shellac and Polyurethane?
You shouldn’t mix shellac and polyurethane. This is because both products have a different chemical formula and paint make-up. Mixing shellac and polyurethane will produce a paint that has varying consistency and color. There are many reasons for this.
For starters, polyurethane is gotten from petroleum residue and thinned with mineral spirits which are also gotten from petroleum distillates. Shellac on the other hand is gotten from the natural secretion of the female ‘lac-bug’ and thinned with alcohol.
This means that both paints not only have a different make-up but are also thinned with different solvents. If mixed, the polyurethane will most likely separate from the shellac.
Also, polyurethane is a sealant or top coat while shellac is commonly used as an undercoat or base coat. This means both paints by design will have different features. Polyurethane on one hand will be designed with extra additives that make the coating water-resistant and tough. Shellac on the other hand will be designed with pigments and paint particles that make the coating attractive.
If mixed and used, the shellac will dry quicker than the polyurethane. This will result in an uneven curing process. Over time, some parts of the paint coating will remain tacky while other parts will have dried.
Polyurethane is also a clear finish while shellac has color. If mixed, the polyurethane will thin the shellac paint. So a good rule of thumb is not to mix shellac and polyurethane.
But if you must for any reason, it’s better to use the jar test first. Add some shellac paint and polyurethane sealant in a clean glass jar. Then watch the reaction of the paints. If the polyurethane floats or separates from the shellac and vice versa, then you can’t mix them.
Can You Apply Shellac Over Polyurethane?
You can apply shellac over polyurethane. This is because shellac paint is designed to have improved paint adhesion. So the shellac paint will adhere to the polyurethane but usually after a light sanding. However, it’s not advised to apply shellac over polyurethane for two main reasons.
First, shellac paint is rigid while polyurethane is flexible. If shellac is used over Polyurethane, it will crack and fall off over time.
Secondly, shellac isn’t strong enough to seal polyurethane. This is because ordinary shellac will be prone to heat and moisture. So the shellac paint will peel off the polyurethane over time.
If you decide to seal the shellac with wax, it will make the finish very blurry because then you would have applied wax over shellac over polyurethane. That will result in too much paint coatings and eventually, a cloudy finish.
It’s not a good idea to apply shellac over polyurethane. Though the shellac will stick to the polyurethane, the finish will not be durable for long. The main reason for this is because shellac is a rigid paint. shellac when dry produces a hard finish that doesn’t expand or contract.
Polyurethane on the other hand is a flexible clear finish. As a plastic, polyurethane can expand and contract in reaction to temperature variations and weather conditions. This is one reason why polyurethane is very durable.
So if you put shellac over polyurethane, the shellac will not cope with the movement of the polyurethane underneath. This will cause the shellac finish to eventually slit open and fall off the polyurethane.
Also, shellac is not as tough as polyurethane so if used over polyurethane, the shellac will be severely affected by grime, dust, heat, and moisture. So never use shellac over polyurethane. Instead, use polyurethane over dewaxed shellac.
Should I Use Wax or Polyurethane Over Shellac?
A common question DIYers ask is if to use wax or polyurethane over shellac. The answer to this question depends on the type of finish you are aiming for and the item to be painted.
If you want a clear finish, then you should go for polyurethane over the shellac. Though wax is also categorized as a clear finish, it is not as transparent as polyurethane. Wax tends to deepen the color of the shellac. Also, wax is thicker than polyurethane so it’s more likely to blur quicker than polyurethane. The clear finish of Polyurethane is better.
If you want an eco-friendly shellac finish, then wax is your pick. Wax is gotten from various natural products including beeswax. This means wax doesn’t produce any fume or toxic odor on the shellac. Polyurethane however is solvent-based and the clear coat has polyols and other chemicals in it that make it more likely to produce an offensive odor.
If you are painting wood that wouldn’t experience heavy usage with shellac, you can use wax over the shellac. Wax though super-tough tends to bleed through coatings. Wax is also known to remain tacky or sticky for days after being applied. So it’s not the best sealant for tabletops, chairs, and countertops.
Polyurethane on the other hand is more suited to furniture and wood that will be used heavily. This includes chairs, desks, cabinets, and slabs. This is because polyurethane doesn’t bleed through coatings and the sealant is super thick and moisture-resistant. So it can handle the extra use, unlike wax.
Overall, you can use polyurethane over shellac to improve the durability of the shellac. Polyurethane also works to get a clear shellac finish. However, you must ensure that the shellac finish is dewaxed before applying polyurethane over it. Remember, polyurethane will not stick to waxed shellac.
Also, don’t apply shellac over polyurethane. The shellac will not last long if used over a sealant like polyurethane.