How to Repair Cracked Polyurethane Wood Coating?

How do you repair a cracked polyurethane wood coating? Why does this happen?

You can repair cracked polyurethane wood coating by stripping the cracked coating by using successive sanding sessions and applying a new coat of polyurethane finish. When repairing a patch on a large workpiece, using the same type of polyurethane as in the existing finish is advised.

In this guide, we tell you the best method we use in our woodworking shop to repair cracked polyurethane coating.

Steps for Repairing Cracked Polyurethane Wood Coating

Comparing the before and after repair condition of a cracked Polyurethane finish
Comparing the before and after repair condition of a cracked Polyurethane finish

Repairing cracked polyurethane wood coatings requires patience, attention to detail, and the right tools and materials.

Follow these steps to repair the coating and restore its original look.

Step 1: Prepare the Surface

Sanding the cracked polyurethane finish
Sanding the cracked polyurethane finish

Before you begin repairing the cracked polyurethane wood coating, it is essential to prepare the wood surface.

This involves removing the old coating, smoothing out the surface, and cleaning it thoroughly.

You can do this by sanding the surface of the wood using sandpaper in various grits, starting with a low grit and working your way up to a high grit.

This will ensure that the surface is free from any dust or debris that could affect the adhesion of the new coating.

Step 2: Fill in any Cracks or Gaps

After preparing the surface, fill in any cracks or gaps in the wood.

To do this, use a putty knife or scraper to apply a high-quality wood filler to the affected areas.

Make sure to choose a wood filler specifically designed for use with polyurethane coatings that matches the natural color of the wood as closely as possible.

After applying the wood filler, smooth it out with the putty knife or scraper, and allow it to dry according to the manufacturer’s instructions.

Step 3: Apply the New Coating

Applying a fresh coat of polyurethane
Applying a fresh coat of polyurethane

Once the wood filler is dry, apply a new coat of polyurethane to the wood surface.

To do this, use a paintbrush or foam brush to apply a thin, even coat of polyurethane coating.

Make sure to choose a polyurethane coating specifically formulated for use with the type of wood you’re working with and matches your desired finish.

Step 4: Sand and Repeat

Sanding between coats of polyurethane
Sanding between coats of polyurethane

After the first polyurethane coat is dry, lightly sand the surface using fine grit sandpaper. This will remove any imperfections and smooth out the surface.

Then apply a second coat of polyurethane and repeat the sanding and coating process until you have achieved the desired level of coverage and finish.

Allow each coat to dry fully before sanding or applying a new coat.

Step 5: Allow the Coating to Cure

Allowing the PU to dry
Allowing the PU to dry

After applying the final coat of polyurethane coating, give it ample time to dry and cure to prevent any cracking or peeling in the near future.

What Causes Polyurethane Finish to Crack?

Cracks in a polyurethane finish
Cracks in an old polyurethane finish

Polyurethane finishes can crack due to various reasons.

Understanding the root causes of these cracks is important to prevent them in the future.

Generally, they can crack due to the following reasons:

Aging and Wear

Over time, polyurethane finishes break down due to exposure to sunlight, moisture, and other natural reasons.

As the coating ages, it becomes less flexible and more prone to cracking, especially in high-traffic areas.

Exposure to Heat and Sunlight

Polyurethane-coated wood in sunlight
Polyurethane-coated wood exposed to sunlight

Polyurethane finishes exposed to high temperatures and direct sunlight may crack due to thermal expansion and contraction.

This is particularly true for finishes applied to exterior surfaces, such as decks and outdoor furniture.

Exposure to Moisture and Humidity

Polyurethane finish exposed to water
Polyurethane finish exposed to water

Polyurethane finishes can also crack when exposed to moisture or high humidity levels.

This can cause the wood to expand and contract, stressing the coating and causing it to crack.

Application Issues

Polyurethane dripping down the wood surface due to over-thinning
Polyurethane dripping down the wood surface due to over-thinning

Polyurethane finishes may also crack if applied incorrectly or if the surface is improperly prepared.

For example, if the coating is too thick, it may dry unevenly and crack.

Similarly, if the surface is not sanded or cleaned properly before applying the coating, the finish may not adhere correctly and may crack as a result.

Substrate Movement and Flexibility

A poly finish can also crack if the substrate (i.e., the wood surface) moves or flexes.

This can occur when the wood expands and contracts due to changes in temperature or humidity or if the surface is subject to heavy loads or impacts.

In some cases, the substrate movement can cause the coating to peel or flake off, leading to further cracking and damage.

Tips for Preventing Future Cracks

Use the Right Type of Polyurethane

Water-Based and Oil-based Polyurethane
Water-Based and Oil-based Polyurethane

When choosing a polyurethane coating for your wood surface, select the right type.

There are two types of polyurethane: oil-based polyurethane and water-based.

Generally, oil-based polyurethane is preferable for outdoor applications due to its excellent resistance to moisture, whereas water-based polyurethane is ideal for indoor applications.

Apply Thin Coats

Applying thin coats of poly using a sponge brush
Applying thin coats of poly using a sponge brush

Instead of applying one thick coat of polyurethane, apply multiple thin coats.

This will help the polyurethane to adhere better to the wood surface and prevent cracking or peeling.

Provide Enough Time for Each Coat to Dry

Using a portable fan to speed up the drying process of polyurethane
Using a portable fan to speed up the drying process of polyurethane

Give enough time for each coat of polyurethane to dry before applying a new coat. This will help prevent the new coat from cracking or peeling.

Furthermore, moisture trapped under a coating can cause bubbling and eventually tear the finish.

Sand Between Coats

Sanding between coats helps smooth out any imperfections and ensures better adhesion of the new coat.

Use fine-grit sandpaper and thoroughly clean the surface before applying the next coat.

Avoid Extreme Temperatures and Humidity

Extreme temperatures and humidity can affect the drying and curing of the polyurethane coating, leading to cracking or peeling.

So apply polyurethane in a well-ventilated area and avoid applying under highly humid or extreme temperature conditions.

Final Thoughts

Polyurethane finish prevents the wood from harmful elements, and a crack in the finish can render the workpiece vulnerable to damage.

However, you can repair cracks in the finish by sanding the cracked finish and applying a fresh polyurethane coating.

Generally, for smaller workpieces, it is advised to sand the entire workpiece and add a fresh coat over the entire workpiece.

In contrast, when repairing a small crack on a large workpiece, it is advised to use masking tape around the cracked area and repair a small patch of the workpiece.

Frequently Asked Questions (FAQ)

How to repair a crack in polyurethane finish caused by the cracking of wood underneath?

To repair a crack in the polyurethane finish caused by the cracking of wood underneath, you will have to strip the polyurethane coating and use wood filler to fix the crack in the base wood and then apply a fresh coating of polyurethane after the wood filler dries.

Can we use heat to self-repair minor cracks in the polyurethane finish?

Yes, you can heat to self-repair minor cracks in the polyurethane finish. However, this method is only suitable for very minor cracks and might still be visible when looked at closely.

Does polyurethane finish provide abrasion resistance?

Yes, polyurethane finish provides good abrasion resistance and prevents the workpiece from scratches that would otherwise damage the workpiece.