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 finish isn’t waxed or has been dewaxed. Polyurethane will protect the 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, so shellac will slide off the finish and won’t stick over it.
Polyurethane sticks to an unwaxed shellac finish because it 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 the sealant.
However, the polyurethane doesn’t stick well if the finish is waxed. That’s because the wax will prevent it from adhering properly. To apply it to stick over waxed shellac, you must sand or remove the wax coating first.
Shellac Dry Time Before Sealant
Shellac must fully dry (cure) for 72 hours (3 days) before sealing it. Curing time refers to the time it takes the solvent to evaporate and the particles to harden. So, it takes shellac 30 minutes to dry enough for a re-coat; this means it’s a quick-drying finish.
However, it takes longer to cure. For shellac to cure (fully dry), the paint solvent must evaporate, and the paint particles must harden. Since it cures through oxidization, it needs 72 hours (3 days) to cure. The dry and cure time depends on the humidity levels, room temperature, and thickness of the coat.
Sealing it too soon 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 finish before sealing it.
Here’s a guide on how to do it:
- Cover Nearby Objects – If you are spraying polyurethane, 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) or sand it. Sanding will remove the wax’s glossy finish and allow it to stick. To sand waxed shellac (for dewaxing), use 150-grit sandpaper.
- Sand The Shellac – If the finish isn’t waxed, sand it with fine-grit sandpaper (220-grit). Fine-grit sandpaper will smooth the coating allowing the sealant to stick better. Don’t use coarse sandpaper as it can remove the finish. After you sand it, remove the dust.
- Apply the Polyurethane – Once the finish is clean and sanded, seal it. 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 Them
You shouldn’t mix shellac and polyurethane. That’s because they have different formulas and aren’t compatible. A mixture of them 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, they will separate from each other. 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, they are different.
Don’t Apply Shellac Over Polyurethane
You can’t apply shellac over polyurethane because the sealant forms a thick glossy finish that prevents moisture from penetrating the finish. Since shellac needs to penetrate the wood grain (surface) to stick and polyurethane doesn’t allow that, it won’t stick.
The only way to apply it is to sand or remove the glossy finish of the sealant. Sanding will create tiny ridges (holes or scratches) that the paint will penetrate. However, the adhesion between shellac and wood won’t be good. So, it’s recommended to remove the entire finish first.
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 because it doesn’t have paint pigments, and the finish is transparent. On the other hand, the wax tends to darken (deepen) the color of the finish. Also, wax is thicker, and it blurs the finish.
- Indoor Surfaces – For indoor surfaces, use wax. That’s because indoor surfaces don’t need protection.
- Outdoor Surfaces – For outdoor surfaces, use polyurethane because it’s 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 to seal shellac to improve the durability of the finish. When dry, it forms a thick glossy finish that is moisture, water, and scratch resistant. However, the finish must be dewaxed or sanded first.
On the other hand, you shouldn’t apply shellac over polyurethane. That’s because the thick glossy finish of polyurethane won’t allow the paint to penetrate or stick.