Polyurethane is a durable clear coat used to seal just about any paint type. But like every other paint, polyurethane will chip after some years. When this happens, can you apply a new coat of polyurethane over the old polyurethane? Here is the answer to that.
You can apply polyurethane over old polyurethane but you will need to sand between coats. Sanding using different grits is required to abrade the old poly so the new polyurethane coating can stick.
That’s not all. There is more to know about applying polyurethane over old polyurethane and thus post digs right into the topic. So, let’s dive in.
Can You Apply A Second Coat Of Polyurethane Without Sanding?
You can’t apply a second coat of polyurethane without sanding the existing coating first. Sanding between coats of polyurethane is required to improve the polyurethane finish and make it bond to the surface.
Polyurethane clear coat especially oil-based polyurethane has a slick texture when dry. This slick texture prevents an additional coat of polyurethane to sit on the existing coat without sanding. Sanding is very important as it helps to scar the existing poly lightly so the new coating can stick.
When you sand the old polyurethane coat using sandpaper, you create tiny scratch marks and dents in the poly.
These tiny holes give the new polyurethane coating something to bite into or hold on to as it is applied over the old poly. Without sanding, you will be applying the new coat over a slick surface and slick isn’t good for paint adhesion.
Sanding the existing coat also helps to remove imperfections and grime on the old poly. Old polyurethane is usually affected by grease, dust nibs, and paint pimples. All of these need to be removed from the old poly as they can negatively affect the smoothness of the new coating.
Also, while applying 3 coats of the new polyurethane on old polyurethane, you need to sand between coats of the new polyurethane. This means you should sand the first coat before applying the second coat.
You should also sand the second coat before applying the third coat. This improves the polyurethane finish and helps with inter-coat adhesion.
As an expert tip, always ensure to allow the existing polyurethane coating to cure before sanding. This can take up to 48 hours. Sanding polyurethane that hasn’t cured will damage the finish.
Can You Put Too Many Coats Of Polyurethane?
You can apply too many coats of polyurethane on a surface. The recommended amount of coats of polyurethane is 3 coats. When you apply more than 3 coats of polyurethane on a surface, you are putting too many coats on that surface
Too many coats of polyurethane will make the polyurethane turn yellow and this will make the finish amateurish.
It is usually advised to stretch out the polyurethane so each coating is thin and fine. Especially when applying polyurethane on either wooden floors, furniture, or metal
You will also need to sand between coats of polyurethane to improve its adhesion. When you stretch out and sand the polyurethane, you are directly removing a part of the applied polyurethane. This is why 3 coats of polyurethane are needed to fill and replace the one you sanded off. This means 3 coats of polyurethane is just about 2.5 coats or less.
However, you can add too much polyurethane to a surface. This usually happens when you add more than 3 coats of polyurethane. Many homeowners believe many coats of polyurethane improves the durability of the finish. This is wrong.
When you add too many coats of polyurethane, you risk the paint turning yellow. This is more common with oil-based polyurethane that has oils and chemicals in the paint.
Too many coats of polyurethane will also make the finish cloudy and the polyurethane finish is meant to be clear. After all, it’s a clear topcoat.
Using more than 3 coats of polyurethane is also a waste of the paint. As stated earlier, sanding removes part of the polyurethane.
This means more coats of polyurethane equals more sanding. Since you have to sand between each coat of polyurethane, you will be sanding off a lot of poly which for the record is an expensive finish.
Related Read: Can You Use Polyurethane on Metal?
How To Apply Polyurethane Over Old Polyurethane
Applying a new coat of polyurethane over old polyurethane is a task that requires enough surface preparation. When the surface is fully prepped, the new coat will stick fine.
To apply polyurethane over old polyurethane, you will need the following tools:
- A Pair Of Work Gloves
- A Can Of Polyurethane Finish
- Fine-Grit Sandpaper
- A Degreaser (Like TSD) Or Mild Solvent
- Paint Thinner
- Drop sheets
- Painter’s Tape
- Aerosol Polyurethane
- Bristled Or Foam Paintbrush
Here is how to apply polyurethane over old polyurethane:
- Clean The Old Polyurethane To Remove Grime
- Sand The Old Polyurethane
- Thin The First Two Coats Of Polyurethane
- Apply The New Polyurethane
- Spray Aerosol Polyurethane On The Finish
Now, let’s get to work.
1. Clean The Old Polyurethane To Remove Grime
The first step before any painting task is to prepare the workspace. Remove any object or furniture that can be affected by paint splatters. Then place a large drop sheet on the floor to cover it. You should also use tape to hold the drop cloth in place.
Painting can be messy, so you need to prep your workspace to handle the mess when it happens. Next, clean the old polyurethane.
The old polyurethane finish would have been affected by dust, grime, and grease. All these will affect the quality of the finish when the polyurethane dries. So you need to remove them first.
To clean the old polyurethane, use a clean rag to wipe the finish. You can also use rubbing alcohol or a degreaser like TSD to remove tough stains.
2. Sand The Old Polyurethane
This next step is very important. Before you can paint on old polyurethane, the old polyurethane finish has to be sanded first. Sanding helps to scar the old polyurethane so the new polyurethane coating can soak into it. This improves paint adhesion and bonding.
To sand the old polyurethane, rub fine-grit sandpaper against the old polyurethane coating. You can start with 150-grit sandpaper before moving on to finer-grit sandpaper-like 320-grit.
Avoid using any sandpaper less than 150 grit on the old polyurethane. This is because polyurethane sands quickly and easily. Using coarse-grit sandpaper on old polyurethane will remove the entire finish.
3. Thin The First Two Coats of Polyurethane
The next step is to thin the first two coats of polyurethane. You usually don’t need to thin polyurethane but the polyurethane finish will take hours to dry before you can apply a new coating. So thinning the first two coats will make the polyurethane dry faster so you can apply the final coat sooner.
Thinning also helps the polyurethane to flow evenly to all nooks and crannies on the surface.
You don’t need to thin the final coat of polyurethane. So pour a bit of polyurethane paint into a plastic bucket and mix with paint thinner.
For oil-based polyurethane, you can use mineral spirit or 100% acetone as the paint thinner. For water-based polyurethane, just use water to thin the paint. You should thin the polyurethane up to 70%. This will fasten the drying time of the poly.
Remember not to thin the whole can of polyurethane. Just pour enough poly to cover the first two coats into a plastic container. Then keep the remaining polyurethane inside its can till you need to apply the final coat.
Related Read: Can Polyurethane Be Tinted?
4. Apply The New Polyurethane Over Old Polyurethane
Once you have stirred the polyurethane and paint thinner inside a bucket, you are ready to apply the paint. To apply polyurethane, it’s best to use a paintbrush or a spray gun.
Use a natural or synthetic bristled brush to apply oil-based polyurethane. Water-based polyurethane can also be applied using a bristled paintbrush or a foam pad. Spray guns also work.
Ensure each coating of the new polyurethane is applied evenly across the old polyurethane.
Also, remember to sand lightly between coats of polyurethane. This helps to improve bonding. When you have applied the first coat, leave it to cure.
This will take a few hours. Then you can apply the second coat and leave it to cure. When the first two coats have cured, you can apply the third and final coat of polyurethane.
The final coat can also be applied with a bristled brush or a foam brush. For the final coat of polyurethane, paint application should be done in long and straight strokes.
Don’t move the brush over the surface repeatedly. This will leave brush marks on the final coat when it dries.
Related Read: Can You Apply Polyurethane With a Rag?
5. Spray Aerosol Polyurethane On The Finish
When the third and final coat of polyurethane has cured, you can spray aerosol polyurethane on it. Aerosol polyurethane is durable, fine, and glossy. It comes in a spray can so you can spray it on the final coat of polyurethane directly. This gives the new polyurethane coating an extra touch of class and durability.
When you are done with all of these steps, clean up your workspace. Then leave the polyurethane to cure for a few days.
This means you shouldn’t touch, use or place any object on the polyurethane finish for at least 3 days. This gives the finish enough time to harden.
Related Read: Can You Mix Latex Paint with Polyurethane?
Can You Apply Exterior Polyurethane Over Interior Polyurethane?
You can apply exterior polyurethane over interior polyurethane as long as the surface or item that is finished will remain indoors.
You shouldn’t use exterior polyurethane over interior polyurethane if you are working on a surface that will be outdoors. This is because the interior polyurethane which serves as the base coat wouldn’t be strong enough to cope with the outdoor weather.
Since the base coat (or interior polyurethane) isn’t strong enough, the whole polyurethane coating will peel off after a while. Even if the topcoat is exterior paint.
Polyurethane like most other paint types has two variations; interior and exterior polyurethane. Both types of poly have similar features. But, exterior polyurethane is designed with additives that make the poly resistant to UV-rays and harsh weather.
Interior polyurethane on the other hand isn’t designed with any additives. So, the paint isn’t resistant to UV-rays or extreme weather conditions. This makes exterior polyurethane tougher than interior polyurethane.
If you use exterior polyurethane over interior polyurethane on a surface or item that will be used outdoors, the result will not be durable.
You may think the exterior polyurethane will protect the interior polyurethane underneath from UV-rays and harsh weather. This is true but exterior polyurethane isn’t 100% resistant to UV rays and moisture.
The constant exposure to UV rays and harsh weather will wear down the exterior polyurethane gradually. This gradual wear of the exterior polyurethane means that the interior polyurethane used underneath will sooner or later be exposed to the UV-rays too.
Since the interior polyurethane underneath isn’t designed to withstand constant exposure to UV rays, it will peel off very quickly. And when this happens, the top coat (or exterior polyurethane) is coming with it.
If you use exterior polyurethane over interior polyurethane on an outdoor surface, the finish at best will last for a year before it begins to fall off.
Related Read: Can You Mix Polyurethane With Chalk Paint?
Can You Put Water-Based Polyurethane Over Old Oil-Based Polyurethane?
It’s possible to put water-based polyurethane over oil-based polyurethane as long as the oil-based polyurethane has been sanded to remove the wax on it.
However, it’s not advised to do so because the water-based polyurethane in most cases wouldn’t stick well to the oil-based polyurethane.
Water-based and Oil-based polyurethane have different chemical formulas and characteristics. Oil-based polyurethane dries to give a glossy and slick surface. This high gloss finish of oil-based polyurethane makes it difficult to use any paint or finish on it. That’s because the paint wouldn’t stick to a slick glossy surface.
So to use water-based polyurethane over oil-based polyurethane, you need to sand the oil-based poly first to get rid of the glossy sheen or wax. When the glossy sheen is off, then the water-based polyurethane can sit on the oil-based polyurethane.
Another thing to know is if you can use oil-based polyurethane over water-based polyurethane. Let’s take a closer look.
Can You Put Oil-Based Polyurethane Over Water-Based Polyurethane?
You can put oil-based polyurethane over water-based polyurethane as long as the water-based polyurethane has completely cured. You will also need to sand the water-based poly lightly to improve bonding.
Unlike oil-based polyurethane, water-based polyurethane doesn’t dry to form a high glossy sheen. The finish of water-based polyurethane though still glossy is more textured than that of oil-based polyurethane. This makes Paint adhesion easier but you still need to sand lightly to improve bonding between the coats of polyurethane.
Asides from sanding, you also need to ensure that the water-based coating has completely cured. This will take weeks or about 2 months. When the water-based polyurethane has cured completely, it just needs to be sanded to make it a suitable base coat for oil-based polyurethane.
If you paint oil-based polyurethane over water-based polyurethane that hasn’t cured, the oil-based polyurethane wouldn’t stick well.
This is because moisture will be trapped inside the water-based coating. The finish can also have a yellowish, whitish, or cloudy appearance.
Related Read: Does Polyurethane Expire?
Overall, you can apply polyurethane over polyurethane as long as you sand the old polyurethane before applying a new coating. Sanding will improve the bonding and quality of the finish.
If you are painting over old oil-based polyurethane, it’s advised to remove the glossy sheen first.