Polyurethane over Old Polyurethane (Can You Do it?)

Polyurethane is a durable clear coat that sticks over any type of paint. But, can you apply polyurethane over an old polyurethane finish?

You can touch up polyurethane with another polyurethane coat. But, you must clean and sand the old sealant to remove the glossy layer it has. Polyurethane will stick regardless if you sand or not, but the adhesion between coats will be better if you sand. 

Also, the type of polyurethane doesn’t matter; oil-based poly will stick over water-based poly (& vice versa). 

Things To Know

Polyurethane is a clear coat that protects the surface underneath from moisture, scratches, and other damage. When dry, it forms a moisture-resistant coating that prevents water from penetrating its surface. Since water (or liquid) can’t penetrate its surface, paint can’t also. That’s why you can’t paint over polyurethane. 

However, polyurethane is a tropical sealant and not a penetrating finish. This means it can stick and dry over a topcoat without penetrating a surface. Because of this, you can apply polyurethane over an old polyurethane coating. The new coating doesn’t need to penetrate the old coating and will stick over it.

Sand or Not?

Sanding between coats of polyurethane is optional, but it helps with paint adhesion. When you sand, you create tiny ridges on the surface that the next coating can penetrate. Also, sanding removes imperfections and bumps and smooths out the surface.

If you want to re-coat a cured (fully dried) polyurethane, sanding will remove the glossy layer of the old sealant, so the new coat can stick better. While this is optional, it does help with adhesion. 

To sand between coats of polyurethane, use fine-grit sandpaper. Fine-grit sandpaper won’t remove the coating but will remove imperfections and even out the surface. To sand old polyurethane before re-coating, use medium-grit sandpaper. The medium-grit sandpaper will remove the glossy layer. 

How To Apply Polyurethane Over Old Polyurethane?

Applying polyurethane over an old polyurethane is easy, but you must prep the coating properly to avoid paint problems. 

Here are the tools you need:

  • Work gloves.
  • Fine-grit sandpaper. 
  • A degreaser (TSD) or mild solvent. 
  • Paint thinner. 
  • Drop sheets.
  • Painter’s tape. 
  • Sprayer or foam paintbrush. 

1. Clean The Old Coating

The first thing you need to do is to clean the old polyurethane coat.

First, clean the old coating to remove dirt, dust, or grime from the surface. To clean, damp a rag with a degreaser (such as TSD) and use the dampened rag to wipe off the stains from the surface. Use a low amount of rubbing alcohol to remove stubborn stains.

2. Sand

Next, sand the old polyurethane. Sanding the old polyurethane allows the new polyurethane to stick better.

To sand old polyurethane, use medium-grit sandpaper until the glossy topcoat is removed. Then, smooth the surface with fine-grit sandpaper. Avoid using coarse-grit sandpaper, as it can remove the entire finish (unless you want that).

After sanding, wipe off the dust from the coating using a clean rag. 

Related Read: Can Polyurethane Be Tinted?

3. Apply Polyurethane

Now, apply the new polyurethane over the old polyurethane.

After the old polyurethane is cleaned and sanded, apply a new coat over it. 

To apply polyurethane, use a paintbrush, rag, or sprayer. If you use a sprayer, you must thin the sealant. To thin water-based poly, use water, and to thin oil-based polyurethane, use mineral spirits. If using a sprayer, thin it in a ratio of 4:1 (4 parts poly, 1 part paint thinner). If using a paintbrush to apply it, thin it in a ratio of 3:1. 

Since there’s already one coat (or two) of polyurethane on the surface, you only need 2-3 new coats. Wait until one coat dries before applying the next one. After applying the final coat, wait 24-48 hours for the polyurethane to fully dry before using the surface. 

Compatibility

You can apply exterior over interior polyurethane. That’s because they have the same formula, except exterior polyurethane has UV-blockers while the interior type doesn’t.

However, you can’t apply interior over exterior polyurethane if the surface is outdoors. That’s because the interior type of polyurethane doesn’t have UV blockers and won’t last long outdoors. But, if the surface is indoors, you can apply interior over exterior poly.

You can also apply water-based over oil-based polyurethane (and vice versa) if the old coating is fully dried (cured).  That’s because when dry, both types have the same features and don’t repel each other.

However, if the old coating hasn’t cured, you shouldn’t apply water-based polyurethane over oil-based polyurethane. That’s because wet oil-based and water-based polyurethane aren’t compatible because they use different solvents (water and oil). If the solvents are mixed, the mixture won’t dry and the finish will peel off.

Related Read: Can You Paint Over Polyurethane?

Final Words

You can apply polyurethane over an old polyurethane, but you must clean and sand the old coating. Cleaning and sanding will remove dust and imperfections from the old coating, allowing the new coating to stick better. 

Tony Adams
Tony Adams

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,

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