Can You Apply Primer Over Polyurethane Paint? (How-To)

I wasn’t feeling the look of the polyurethane finish on my kitchen cabinet. So I decided to apply paint over it but I needed to prime it first. So I was wondering, can you prime over polyurethane? I made some research and here is the answer to that.

You can prime over polyurethane. This is because paint primers are designed with resinous compounds that improve paint adhesion and make the primer stick to virtually any surface including polyurethane.

But you need to prep the polyurethane by sanding first. Also, you should only use oil-based primer paint over polyurethane. Water-based and acrylic primers will not adhere well to polyurethane.

But there is more to know about painting over polyurethane and this post digs right into the topic. So let’s dive in.

Does Polyurethane Paint Need a Primer?

To seal a painted surface with polyurethane, you don’t need a primer. This is because the paint used on the surface will serve as a good base coat for the polyurethane application.

You also don’t need to put primer on wood before sealing with polyurethane unless you want to alter the color of the wood.

However, to paint over polyurethane, you need a primer first. This is because polyurethane has a very thick and glossy finish that prevents paints regardless of the type from adhering properly. If you paint over polyurethane without priming first, the paint will peel off in weeks.

Polyurethane is a very strong and sturdy finish. It is used to seal virtually any type of surface. But using a primer before applying polyurethane is not necessary unless you are painting over a rough surface. Polyurethane can be used directly on wood as long as the wood is very smooth and properly sanded.

Polyurethane can also be used on paint without priming first. After painting walls and furniture, painters usually seal the painted item directly with polyurethane to protect the finish.

However, if you want to paint over polyurethane, you need a primer. Priming is necessary to paint over poly. This is because poly when dry is impermeable meaning that it is non-porous.

The polyurethane finish is also glossy. The characteristics of the polyurethane finish swill prevent paints from sticking to it. So you need to primer first. Primers are designed with resins that allow paints to stick to surfaces they wouldn’t stick to ordinarily like polyurethane.

Do I Need to Prime Before Polyurethane?

Do I Need to Prime Before Polyurethane?

You don’t need to prime before applying polyurethane unless you are painting a slick material like plastic. This is because polyurethane will stick to material or paint without priming first as long as the material is well sanded.

Also, if the material to be painted is rough or uneven. you need to prime before applying the polyurethane. If polyurethane is applied over a rough surface, the finish will not come out smooth.

Ordinarily, priming before applying polyurethane is not necessary. The common reason many people prime before applying polyurethane is to alter the look of the wood or material. Though not necessary, priming before applying polyurethane will help to improve the smoothness of the finish.

Polyurethane is a very common sealant used on various materials. It is thick, moisture-resistant and exterior polyurethane is even UV-resistant. So it’s a perfect sealant but like most sealants, polyurethane had no color. This is because the sealant doesn’t have paint pigments in its formula that will produce color when used.

So before applying polyurethane to a material, many painters and DIYers use a primer first. It’s also very common to use latex paint or acrylic color on the material first before applying polyurethane. The paint used will serve as the base coat or primer giving the polyurethane finish color.

Asides from the color tint, it’s also common to prime before applying polyurethane when working on a rough material. So for rough surfaces and to give the Polyurethane color, you should prime first.

Though priming before applying polyurethane isn’t always necessary, the reverse is the case when you want to paint over polyurethane. To paint over poly, priming is very necessary. Let’s check out how to prime over polyurethane.

How to Prime Over Polyurethane?

Priming over polyurethane can be a bit time-consuming because the polyurethane surface needs to be well sanded to allow the primer to sit. Polyurethane however is not the easiest of finishes to sand.

Here is a quick rundown of how to prime over polyurethane:

1. Wipe the Polyurethane Finish With Clean Lint-free Rags

This is to remove grime and grease on the polyurethane. You can also use a degreaser or mild solvent like white spirit to clean the polyurethane.

2. Fill the Cracks and Holes in the Material With Wood Putty

This is a paste-like compound that helps to seal holes and cracks in the wood. For concrete, use a concrete filler.

3. Sand the Polyurethane With Ultra-fine Sandpaper

This includes sandpaper from 320 grit and above. This is to abrade the top layer of the polyurethane to allow the primer to sit properly. If the polyurethane isn’t sanded, the top glossy layer will not allow the primer to adhere properly.

4. Remove Dust

After sanding, you’ll notice sanded dust on the polyurethane. Vacuum or wipe the dust.

5. Apply Two Coats of Oil-based Primer on the Polyurethane

The best primer to use over polyurethane is oil-based primer. If you plan on using water-based paint on the primer, ensure to use an oil-based primer that is compatible with water-based paints.

After applying the first coat of the primer, let it dry fully. Then apply the second coat. You don’t need to sand between coats of primer. When the primer is dry, you should…

6. Apply the Required Paint and Let It Dry for at Least 48 Hours Before Using It

So now you know that you can primer over polyurethane. But can you apply polyurethane over prime? Let’s find out.

Can You Apply Polyurethane Over Primer?

You can apply polyurethane over a primed surface. This is because polyurethane will stick to primer. However, you need to ensure that the primer used is compatible with the polyurethane.

The best primer to use before applying polyurethane is oil-based primer. Water-based primers work too but are not as good as oil-based primers. Also, water-based primers are more ideal for latex polyurethane.

Polyurethane will benefit if used over a primer especially on slick surfaces like plastic. This is because paint primers have resins that react to form a smooth surface that will improve paint bonding.

When painting over surfaces like glass and plastic with Polyurethane, it’s always advised to prime first. This is because these surfaces don’t have any fiber or grain that the polyurethane will bond to. But if you prime first, the primer will provide a good base coat that the polyurethane can bite into.

Also, ensure to sand primer before using polyurethane. Speaking of sanding, can you prime over polyurethane without sanding? Let’s find out.

Can You Prime Over Polyurethane Without Sanding?

You shouldn’t prime over polyurethane without sanding. This is because the primer will not stick to the Polyurethane if the poly isn’t sanded first. Also, if you prime over polyurethane without sanding first, the overall finish will peel off.

Polyurethane as explained earlier is a very strong finish. Asides from being strong, polyurethane is also moisture-resistant. The polyurethane film when dry prevents moisture and even paints from penetrating.

If moisture can’t penetrate the polyurethane, it means that primer if applied over polyurethane will not bite into the polyurethane. Instead, the primer will just sit on the top slick layer of the polyurethane.

If the primer is just sitting on the Polyurethane, it means the primer coating will not adhere well. This is why sanding is important before priming over polyurethane.

When you sand polyurethane, what you are doing is slightly buffing the polyurethane finish. You are also removing the top layer of the polyurethane finish that will not let paint stick.

The grits in the sandpaper also create tiny marks and scratches in the polyurethane coating. These tiny marks allow the primer to penetrate and bite into the polyurethane coating to improve adhesion. Sanding before priming over polyurethane is very necessary.

What Happens if You Don’t Sand Before Priming Over Polyurethane?

If you don’t sand before priming over polyurethane, the primer and successive coats applied over the primer will not be durable. The whole paint coating will peel off with minimal use.

As explained earlier, the purpose of sanding Polyurethane before priming over it is to improve paint bonding. Polyurethane finish is too thick and hard for any paint including primers to adhere to it. So sanding helps to create pores in the polyurethane that paints will bite into.

If you don’t sand before priming polyurethane,

  • The finish will come off easily especially if used on surfaces that will see a lot of action like tabletops and countertops.
  • The finish will not be smooth:  Sanding doesn’t just improve paint bonding. It also helps to remove dust nibs and grain on the polyurethane. If you don’t sand, these imperfections will prevent even application of the primer, and the resulting finish will be rough.
  • The finish may remain tacky or sticky for days:  This means the finish will not dry quickly and if this happens, the topcoat will be easily scarred by fingernails, cutlery, and the likes.
  • The finish can be dragged off:  Since the poly wasn’t sanded, the whole finish will not adhere properly. When you decide to clean the finish, you can easily drag the finish of the material since there is no good bonding.

Final Words

Overall, you can prime over polyurethane. Not only is this task possible, but it’s also necessary when you want to paint over polyurethane. If you don’t prime over polyurethane, paints will not stick.

Remember, you should also ensure to sand the polyurethane before applying primer over it. Just as primer improves paint bonding on polyurethane, sanding also improves primer bonding on polyurethane.

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