Can You Paint Over Polyurethane?

What Is Polyurethane?

Polyurethane is a versatile polymer material that is made by reacting a polyol with an isocyanate. The resulting product is a strong, durable material that can be used in a wide variety of applications.

Polyurethanes are known for their flexibility, toughness, and resistance to water, chemicals, and heat. They are also relatively lightweight and easy to work with.

These properties make polyurethanes ideal for a wide range of applications, including:

• Foams: Polyurethane foams are used for cushioning, insulation, and soundproofing. They are found in mattresses, furniture, refrigerators, and other products.
• Coatings: Polyurethane coatings protect surfaces from corrosion, wear, and moisture. They are found on cars, boats, appliances, and other products.
• Adhesives: Polyurethane adhesives bond various materials, including wood, metal, and plastic. They are used in construction, manufacturing, and other industries.
• Elastomers: Polyurethane elastomers are used to make products that need to be flexible and resilient, such as tires, gaskets, and seals.
• Fibers: Polyurethane fibers are used to make products that need to be strong and lightweight, such as Spandex and Nomex.

There are two main types of polyurethane used in woodworking:

• Water-based polyurethane: This type of polyurethane is easier to apply and dries faster than oil-based polyurethane. It is also less toxic and has a lower odor. However, it is not as durable as oil-based polyurethane and may not be as resistant to scratches and fading.
• Oil-based polyurethane: This type of polyurethane is more durable and resistant to scratches and fading than water-based polyurethane. However, it is also more difficult to apply and dries more slowly. It also has a stronger odor and may not be as safe to use indoors.

We mention these because the type of polyurethane we are going to paint over will determine the type of paint we will use.

Prepping A Polyurethane Surface For Painting

As we have stated, polyurethane dries to a smooth finish – with the application of 4-6 coats, that is, with light sanding in between coats and allowing each coat to dry before adding the next.

And we know that paint needs something to latch onto when applied, and a smooth polyurethane finish doesn’t supply that.

Something needs to be done to the polyurethane surface so the paint will adhere.  For this task, there are several options to consider.

Sanding Polyurethane Before Painting

Sand the surface. Use a fine-grit sandpaper to sand the polyurethane surface. This will help to create a rough surface that the paint can adhere to, as we know it will not adhere to a smooth surface.

Of course, you could simply sand it down to the wood, removing all poly, but that is not necessary.  Simply creating a rougher texture over the entire surface to be painted will do the trick for you, something the paint can grab hold of and stay.

It will be easy if the wooden surface you wish to paint is even, i.e., no curves or angles.  If you have curved pieces on that piece of furniture you wish to paint, there are contour sanding grips that can help.

Your Dremel can help with concave curves if you use a drum sander attachment.  Otherwise, on an even surface, your random orbital sander with a high-grit paper will give you what you want. Either way, though, a little elbow grease will be needed.

After sanding, the same rules apply to all other sanding tasks. Use a clean cloth or tack cloth to wipe away the dust and give the surface a good cleaning

Liquid Sanders On Polyurethane Before Painting

Liquid sandpaper is for those who want something a little easier than hand or power tool sanding, and dealing with curvy legs or concave spots, those intricate details on furniture that hand sanding and power tool sanding just don’t reach.

Your piece may be large, too, and you want an easier solution.  Liquid sandpaper can handle those chores for you.

It will scratch the polyurethane surface to rough it up enough for the paint to adhere.

Liquid sandpaper, though, is a chemical and a bit harsh, in fact.  So, safety first rules apply, and protective goggles, protective clothing to cover your skin, and rubber gloves are necessary.

Work in a well-ventilated area, too.  The chemical works like a fine-grade sandpaper, though, and will leave you a paintable surface.

A deglosser like Rustoleum Cabinet Transformation can also help.  That’s the product I am leaning toward for my kitchen cabinets.  I plan to test it on an inconspicuous area first, and you might want to do the same to ensure it’s the right product for me in my project.

It actually comes in a kit, and you can find a variety of them (depending on the color and such you’ve chosen) for between $40 – $60.

Using Bonding Primers on Polyurethane Before Painting

Don’t feel like sanding or using a liquid sander?  Looking for a more direct approach?  Why not try a bonding primer?

A good quality primer can go a long way to prepare the polyurethane for painting and can even lower the number of coats necessary for a good finish.  There are several good ones to choose from, each of which we’ve used in past projects, one of which involved painting red bricks.  

You can “bin it,” as we say.  A Zinsser B-I-N primer works well to prepare a surface for paint application.  You can apply it with a paint brush, bristle, or foam, will work.  BIN also will protect your work from tannin bleed (oak often will bleed some of its tannin) that can mar an otherwise good paint job.

You’ll want some mineral spirits handy, as mineral spirits will clean up any BIN spill or drippings during application. 

A water-borne bonding primer like INSL-X STIX will work well over poly, too.  You can use it both on interior and exterior projects, as well.  It’s as easy to apply as BIN – a paint brush will do.

If you use it, give it a very light sanding with high-grit sandpaper before painting.  It sticks to anything, though, and gives good adherence to the paint.

What Types Of Paint Should You Use Over Polyurethane?

Now that the prep work is done and the work piece is ready for paint, the next question is what type of paint to use over the polyurethane.

Before you paint, though, give the furniture piece or cabinet a good cleaning.  Again, mineral spirits and a clean cloth are recommended as this final prep step.  A cleaned surface will give you a better paint job finish and save you from having to sand it after the first coat of paint.

Obviously, my kitchen cabinets will have some grease I’ll want to remove, too, with a bit of dish soap and warm water, finishing with a dry cloth to get the surface ready.  

We know we have both oil-based paint and water-based paint to choose from, but does it matter which one we use?

The general rule is if you are using an oil-based primer following the prep work, use an oil-based paint.  Oil-based primers are less expensive than the paint you will be using, so it’s a cost-effective measure that will result in a better paint job.

A water-based primer is the choice if you are using a water-based paint.

For my kitchen cabinets, I’m considering a chalk paint or alkyd paint.  We wrote an article on a variety of paints, including chalk paints and alkyd paints, in fact.  The question in that article was about the use of “wall” paints on furniture.

The lower-priced “wall” paints will be latex paint, water-based, while the higher-priced “wall” paints will be acrylic paint.  Acrylic paints have an advantage in requiring little surface preparation work on the wood surface and are self-leveling (so no worry about brush marks).

But I am still leaning toward chalk paint or alkyd for my kitchen cabinets.

Polyurethane Over Paint

How about applying polyurethane over paint?  Sure, and no prep is necessary other than to let the paint dry thoroughly before doing so.  

Yes, polyurethane can be used over a painted surface.  A coat of polyurethane over the paint will provide a durable finish over the paint to protect the painted surface from damage.

We just thought we’d address the reverse – poly over paint – since we had discussed at great length paint over poly.

It can be done and often is.

Adequate prep work will ensure a good paint job.  As with all woodworking projects, patience is required to get a good result.  If you take the necessary steps to prepare the polyurethane surface to accept paint, you’ll get a good result.

Wish me luck on my kitchen cabinet do-over. Maybe we’ll take pictures, describe the process, and write a future article on the project.