Usually, polyurethane is the final coat in a painting project. But, can you apply varnish over polyurethane?
You can apply varnish over polyurethane, but you shouldn’t. That’s because varnish is made of a few penetrating finishes and needs to penetrate the surface to stick. While it can also stick without penetrating, the adhesion will be weak.
When dry, polyurethane forms a clear coat that repels moisture and prevents liquid from penetrating its coating. So, if you apply a penetrating finish over it, polyurethane won’t allow it to penetrate. So, you must sand to create tiny ridges (pores) that the varnish can penetrate.
Varnish and polyurethane are both topcoats (used as the final coat in painting projects); they have some similar features but aren’t compatible.
Varnish contains some penetrating finishes, such as linseed oil, on its formula, so it needs to penetrate the surface (just a bit) to stick. While all paints allow liquid to penetrate its surface (just a bit), polyurethane doesn’t. So, these two topcoats aren’t compatible.
The only way to apply varnish over polyurethane is to sand the surface. Sanding will create tiny ridges (pores) in the polyurethane coating that the varnish can penetrate.
If you don’t sand, there won’t be any space (or pores) for the varnish to penetrate through, meaning there will be a weak bonding between both finishes.
Related Read: Polyurethane vs Varnish?
How To Apply Varnish Over Polyurethane?
Applying varnish over polyurethane isn’t easy. You must sand the surface just enough to allow penetration.
Here are the tools you need:
- Varnish (preferably water-based varnish)
- A bristled paintbrush
- A pair of gloves
- Paint Thinner
- A face mask
1. Clean The Coating
First, wipe and clean the polyurethane coating. Cleaning will remove dust nibs and grain that can clog the sandpaper. Also, if the coating is filthy or dusty, varnish won’t stick over it.
To clean a polyurethane coating:
- Wipe the coating with a clean rag.
- Apply a degreaser (that is paint-friendly).
- Use a soft brush to wipe off the stains and dirt.
- Apply clean water to remove the degreaser residue.
- Allow the coating to dry.
2. Sand The Polyurethane
Once the coating is clean and dry, sand it. Sanding will create tiny pores in the polyurethane that varnish can penetrate into.
To sand polyurethane before varnish:
- Use medium-grit sandpaper.
- Don’t apply pressure while sanding, as you can remove the coating.
- Use fine-grit sandpaper to smooth out the coating.
- After sanding, remove dust from the coating.
Avoid using coarse-grit sandpaper, as you can remove the coating.
3. Thin The Varnish
Varnish has a thick flow, and it’s recommended to thin it. A thin coating is easier to control, dries fast, and provides a smooth finish. To thin water-based varnish, use ½ cup of water. To thin oil-based varnish, use mineral spirits or lacquer thinner.
To thin varnish:
- Use water to thin water-based varnish. Use mineral spirits or lacquer thinner to thin oil-based varnish.
- Stir the mixture with a turning stick or paint mixer for 5-10 minutes.
- Don’t stir too hard, or fast as you can create bubbles.
- Test the mixture.
4. Apply The Varnish
To apply varnish:
- Use a bristled paintbrush, as it allows you to control the coating better.
- Since there are already a few coats of polyurethane underneath, you only need 2 coats of varnish.
- Wait until one coat dries before applying the next one.
- It takes varnish 4 hours to dry enough for a re-coat.
- Wait 24 hours after applying the final coat before using the surface.
Mixing Both Finishes
You shouldn’t mix varnish and polyurethane because they have different features. Polyurethane has a plastic-like finish that forms over a surface without penetrating it. In comparison, varnish has a clear moisture-resistant coating that must penetrate a surface to stick. Since they have different features, they aren’t compatible and shouldn’t be mixed.
However, you can add a small amount of acrylic varnish to water-based polyurethane to create a colorful finish. Acrylic varnish is mixed with acrylic paint and has lots of paint pigments. Since water-based polyurethane is a clear coat (it has no paint pigments or color), you can mix acrylic varnish with it to create a colorful finish. Also, these two finish types use water as their solvent and are compatible.
Polyurethane over Varnish
You can apply polyurethane over varnish. That’s because polyurethane is a sealant and can be applied over all types of finishes because it doesn’t need to penetrate their surface. Doing so will create a super durable finish that protects the surface underneath from moisture, scratches, handling, or other damage.
However, the finish will be too thick and might become blurry. That’s because both finishes have a clear (transparent) finish, and the more coats you add, the blurrier the finish gets.
In summary, applying varnish over polyurethane is not recommended because it won’t stick properly. That’s because polyurethane prevents liquid from penetrating its coating, and varnish must penetrate (just a bit) the surface to stick. So, these two finishes aren’t compatible.
Tony is a professional painter and an author of DIY Geeks. Tony has completed over 1,000 painting projects for his clients. It's safe to say he knows what he Is talking about,