Polyurethane is a clear coat that protects wood from moisture and water. In comparison, shellac is a waxed finish known for its smoothness and color. So, can you apply polyurethane over shellac?
You can apply polyurethane over shellac, but only if the shellac isn’t waxed or has been dewaxed. Polyurethane will protect the shellac finish from moisture, water, and scratches.
On the other hand, you shouldn’t apply shellac over polyurethane. That’s because polyurethane forms a glossy finish that repels moisture, including shellac. So if used, shellac will slide off the finish and won’t stick over polyurethane.
Polyurethane Sticks To Shellac
Polyurethane sticks to an unwaxed shellac finish. That’s because polyurethane doesn’t need to penetrate a surface to stick.
Also, ordinary or unwaxed shellac doesn’t have a topcoat that prevents paints from bonding to it. Shellac is gotten from natural secretions, meaning there’s nothing on the formula that repels polyurethane.
However, the polyurethane doesn’t stick well if the shellac is waxed. That’s because the wax will prevent the polyurethane from adhering properly. To allow polyurethane to stick over waxed shellac, you must sand or remove the wax from the shellac.
Shellac Dry Time Before Polyurethane
Before applying polyurethane, shellac must dry for 72 hours (3 days). Shellac paint must cure before you can seal it. Curing time refers to the time it takes shellac solvent to evaporate and the paint particles to harden.
For the shellac finish to dry, the paint solvent must evaporate. So, it takes shellac 30 minutes to dry enough for a re-coat; this means shellac is a quick-drying finish.
However, shellac takes longer to cure. For shellac to cure (fully dry), the paint solvent must evaporate, and the paint particles must harden. Since shellac cures through oxidization, it needs 72 hours (3 days) to cure. The shellac dry and cure time depends on the humidity levels, room temperature, and thickness of the coat.
Applying polyurethane before shellac fully dries will make the finish sticky. That’s because polyurethane has a plastic-like finish that doesn’t allow anything to go through, especially the solvent. So, if the polyurethane prevents the solvent of shellac from evaporating, the finish won’t dry and turn tacky.
How to Apply Polyurethane Over Shellac?
It’s not hard to apply poly over shellac. You just need to clean the shellac finish and apply the polyurethane.
Here’s a guide on how to do it:
- Cover Nearby Objects – If you are spraying polyurethane on shellac, cover nearby objects and the floor. This is to prevent any paint spills or unwanted spray on your furniture.
- Dewax Shellac – Polyurethane won’t stick well to waxed shellac. So, you must dewax (remove the wax) shellac or sand it. Sanding will remove the wax’s glossy finish and allow the polyurethane to stick. To sand waxed shellac (for dewaxing), use 150-grit sandpaper.
- Sand The Shellac – If the shellac isn’t waxed, sand it with fine-grit sandpaper (220-grit). Fine-grit sandpaper will smooth the shellac coating allowing the polyurethane to stick better. Don’t use coarse sandpaper as it can remove the shellac finish. After you sand shellac, remove the dust.
- Apply the Polyurethane – Once the shellac finish is clean and sanded, apply polyurethane. For outdoor shellac surfaces, use oil-based or exterior polyurethane. For indoor surfaces, use water-based polyurethane. Apply 2-4 coats of polyurethane on shellac using a paintbrush or sprayer. Wait until one coat of poly dries before applying the next one.
Don’t Mix Shellac With Polyurethane
You shouldn’t mix shellac and polyurethane. That’s because shellac and polyurethane have different paint formulas and aren’t compatible. A mixture of shellac and polyurethane will produce a finish that has an inconsistent flow and color.
Polyurethane is gotten from petroleum residue and thinned with mineral spirits, which are also gotten from petroleum distillates. On the other hand, shellac is gotten from the natural secretion of the female ‘lac-bug’ and thinned with alcohol.
So, if you mix both finishes, polyurethane will separate from shellac. This means one part of the mixture will dry while the other part will still be wet. This leads to inconsistent dry time and paint cracks.
Also, polyurethane is a topcoat, while shellac is used as an undercoat. So, by design, polyurethane and shellac are different.
Don’t Apply Shellac Over Polyurethane
You can’t apply shellac over polyurethane. That’s because polyurethane forms a thick glossy finish that prevents moisture from penetrating the finish. And, shellac needs to penetrate the wood grain (surface) to stick. Once the shellac penetrates the finish, it will raise the wood grain and start to dry.
So, since polyurethane doesn’t allow penetration of the surface and shellac needs to penetrate to stick, the shellac won’t stick over polyurethane.
The only way to apply shellac over polyurethane is to sand or remove the polyurethane. Sanding will create tiny ridges (holes or scratches) that the shellac will penetrate. However, the adhesion between shellac and wood won’t be good. So, the best way to apply shellac over polyurethane is to remove the polyurethane.
Wax vs Polyurethane (For Shellac)
Since shellac isn’t moisture-resistant or durable, you must seal it for outdoor surfaces. But, should you use wax or polyurethane?
- For a Clear Finish – If you want a clear finish, use polyurethane over shellac. That’s because polyurethane doesn’t have paint pigments, and the finish is transparent. On the other hand, the wax tends to darken (deepen) the color of the shellac. Also, wax is thicker than polyurethane, and it blurs the finish.
- Indoor Surfaces – For indoor surfaces, use wax. That’s because indoor surfaces don’t need protection.
- Outdoor Surfaces – However, for outdoor surfaces, use polyurethane. That’s because polyurethane is more durable than wax and will protect the surface better. Also, the wax tends to bleed through the coating and remain tacky for days.
You can use polyurethane over shellac to improve the durability of the finish. When dry, polyurethane forms a thick glossy finish that is moisture, water, and scratch resistant. However, the shellac must be dewaxed or sanded for polyurethane to stick.
On the other hand, you shouldn’t apply shellac over polyurethane. That’s because the thick glossy finish of polyurethane won’t allow the shellac to penetrate or stick.