Polyurethane Over Chalk Paint (Can You Do it & How To?)

Chalk paint isn’t durable or moisture-resistant because it’s water-based. So, can you apply polyurethane over chalk paint?

You can apply polyurethane over chalk paint. Polyurethane will produce a glossy moisture-resistant layer that will protect chalk paint from moisture, water, scratches, and other damage. 

For indoor surfaces, sealing chalk paint is optional. But, for outdoor surfaces, sealing chalk paint with polyurethane (or other sealants) is mandatory. That’s because chalk paint isn’t durable enough to withstand outdoor elements. 

Types of Polyurethane To Use on Chalk Paint

You can use three types of polyurethane over chalk paint: oil-based, water-based, and exterior polyurethane.

Oil-Based & Exterior Polyurethane

For outdoor surfaces, oil-based or exterior polyurethane is better for chalk paint. That’s because oil-based and exterior polyurethane are more durable than water-based polyurethane.

However, applying oil-based polyurethane is harder. That’s because oil-based polyurethane takes longer to dry and is thicker than water-based polyurethane. In addition, you must thin oil-based polyurethane before applying it over chalk paint. 

Oil-based polyurethane tends to yellow over time and stain a white-painted surface. 

Water-based Polyurethane

For indoor surfaces, use water-based polyurethane for chalk paint. That’s because water-based polyurethane dries faster, is easier to apply, and you don’t need to thin it. Also, water-based polyurethane doesn’t yellow over time. 

Chalk Paint Dry Time Before Polyurethane

Chalk paint needs to cure for 24 hours before you apply polyurethane. If you apply polyurethane before chalk paint cures (fully dries), the sealant won’t stick, and the finish will turn sticky. 

On average, chalk paint dries for a re-coat within 30 minutes. However, chalk paint must cure (oxidize) for 24 hours before you can apply a sealant (polyurethane) over it. That’s because once chalk paint cures (fully dries), the paint particles harden and compact, meaning chalk paint will be strong enough to handle a sealant. 

However, the dry and cure time of chalk paint depends on room temperature, humidity levels, surface type, and thickness of the paint. You can speed up the chalk paint dry time by increasing air circulation and improving the evaporation rate. 

How To Apply Polyurethane Over Chalk Paint?

Polyurethane sticks over all types of paint. So, applying polyurethane on chalk paint isn’t hard. Here’s how to do it:

1. Clean The Surface

First, clean the chalk-painted surface to remove stains and dirt. The chalk paint surface must be clean and even because polyurethane is a clear coat that shows imperfections. So, if you don’t clean the surface, imperfections will show once the polyurethane dries. 

To clean a chalk paint surface:

  1. Mix soap with warm water.
  2. Apply the mixture over chalk paint.
  3. Use a soft brush or clean rag to scrub the dirt or stains off. 
  4. Remove the soapy water residue from the surface using clean water. 

Before cleaning it, you wait until the chalk paint fully dries (cures). If you clean chalk paint before it dries, you will remove the paint. Also, if there aren’t stains or dirt over the chalk paint, use a clean rag (without water) to remove the dust.

Related Read: Will Polyurethane Stick to Tacky Stain?

2. Sand Down The Surface

Sanding chalk paint before polyurethane is optional because polyurethane will stick regardless. However, sanding will remove bumps and imperfections from the surface. Also, sanding will leave a smooth and even surface behind for polyurethane to stick.

To sand chalk paint, use fine-grit sandpaper (220-grit). Don’t use lower-grit sandpaper such as 80 or 120-grit, as you can remove the chalk paint. 

After sanding chalk paint, remove the dust. 

3. Apply The Polyurethane

Once the chalk paint is clean, sanded, and dry, apply polyurethane. 

To apply polyurethane, use a paintbrush, roller, or sprayer. If you use a sprayer, you must thin polyurethane. Apply 2-3 coats of polyurethane. Wait until the previous coat dries before applying the next one. 

Optionally, you can sand between coats of polyurethane (except for the final coat). 

Polyurethane vs Wax (For Chalk Paint)

Polyurethane is easier to apply and maintain on chalk pants than wax. Wax will give chalk paint finish a darker shade, while polyurethane won’t change the color shade of the finish. 

For high-traffic surfaces, polyurethane is better than wax. That’s because the wax is a low durability finish that can’t withstand high use. If the wax finish gets damaged, you must re-apply the wax to fix it. And, re-applying wax over chalk paint is hard because you must remove most of the wax. 

On the other hand, polyurethane is a durable finish that won’t get damaged easily. Polyurethane will last 5-10 years over a chalk paint surface if you maintain it properly. Also, if the polyurethane coating gets damaged, all you need to do is sand the coating and re-apply a new one. 

Mixing Chalk Paint With Polyurethane

You can mix water-based polyurethane with chalk paint. The mixture will have a colorful and durable finish. However, you can’t mix oil-based or exterior polyurethane with chalk paint. That’s because chalk paint uses water as its solvent, and oil-based polyurethane uses oil. And oil and water don’t mix. 

You must only mix water-based polyurethane with chalk paint to tint the polyurethane finish. Polyurethane has a clear finish (no color) because it doesn’t have paint pigments in its formula. So, if you mix chalk paint and polyurethane, the finish will be colorful. That’s because chalk paint has lots of paint pigments in its formula.

However, the best paint to mix with polyurethane is acrylic paint. That’s because acrylic paint has the most paint pigments of all paints. 

Conclusion

Applying poly over chalk paint is necessary for outdoor surfaces. When dry, polyurethane will form a thick glossy finish that is moisture, water, and scratch resistant. In addition, the glossy finish of polyurethane will protect the chalk paint and the surface underneath from damage.

However, before applying polyurethane over chalk paint, the paint must cure (fully dry).

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