Can You Put Polyurethane Over Old Polyurethane?

Image of polyurethane floor but Can You Put Polyurethane Over Old Polyurethane?You’ll want to have comfortable furniture in your home without sacrificing beauty. Woodworkers prefer using polyurethane for furniture and wooden floor finishing, and many more. Polyurethane is a choice of many because it’s a flexible foam, and it’s one of the most popular materials for home finishings. It comes with cushioning features, suitable for upholstered furniture. This product has increasing impact resistance, load-bearing capacity, and excellent wear properties. However, can you put polyurethane over old polyurethane? 

Yes, you can apply polyurethane to your old polyurethane finished hardwood if you use the proper steps to prepare the finish. You have to start by lightly sanding with a dull 120 to 150 grit abrasive screen or a No.2 steel wool using a heavy floor buffer. If you want to sand the edges, corners, and beneath cabinet toe kicks:

You will realize that polyurethane comes as either oil or water-based. Even though oil-based is commonly used, when using water-based polyurethane, consider applying with a unique applicator. Besides, it would be great if you consistently applied the finish in the same direction as the wood grain.

Can You Apply a Second Coat of Polyurethane Without Sanding?

No, you can’t. There is no substitute for a better finish without sanding between each coat. When applying two to three coats of full strength polyurethane, ensure you sand between each coat. If you want to stretch the polyurethane into a thin coat, you should brush along the grain. Before sanding, allow it to dry at least between 24 to 48 hours. Within this time, the surface will have enough time to cure and then harden.

Facts About Polyurethane

  • Brush across the wood grain first to work the finish into the wood

The good thing about all finishes is that they ideally soak into the wood no matter how you apply. They do this with the help of capillary action. The benefit of brushing across the grain to work the finish into the wood is to make the varnish thickness more even. However, the longer you brush, the thinner it evaporates. Therefore, if you keep brushing, the finish will thicken, and the brush marks will be more pronounced.

  • Thin the first coat 50% to get a good bond

Mostly, primers create a better bond for paint due to their higher ratio of binder (finish) to pigment. However, all finishes are binders. Thus they can bond perfectly without a particular product. 

When using polyurethane, you can attain perfect bonding by thinning the first coat up to 50%. Through this, the surface will have a fast drying time. The thinner the layer of all kinds of finish, the faster the drying time. Therefore, you can sand and apply the next coat sooner. 

  • It may form bubbles.

If you shake the can before applications, bubbles will appear in the finish. Also, when you brush the finish, bubbles will appear in it. However, a great majority of bubbles are a result of brushing. You can avoid bubbles in the final finish by reducing the turbulence caused by the rapid movement of the brush. 

You should note that you’ll always get bubbles whether you shake or don’t shake the can. Therefore, what you must know is how to keep the bubbles from drying in the finish. Mostly, bubbles pop out on their own. 

Woodworker using Rigid, the Best Cordless Random Orbital SanderIf they get trapped, you may help the popping by tipping them off using your brush. If this process doesn’t work out well, you can add 5% or 10% mineral spirits so the finish can stay open for longer and allow the bubbles to pop out.

  • Scuff and sand between coats to get a good bond

The primary purpose of sanding between coats is to create scratches on the surface. Therefore, it allows the next coat of finish to establish a mechanical bond. You should also note that polyurethane dries slowly. Thus, there are dust nibs which you should sand out before applying the next coat. 

Use sandpaper grit that can remove all the flaws effectively without creating deeper scratches. I would recommend sanding with a #320 or #400 grit, regular or “P” grade sander. If you decide to use a flat block, your sandpaper will clog, putting deeper scratches into the finish.

How Do You Restore a Polyurethane Finish?

Restoring your polyurethane finish is as easy as dipping finer sandpaper or a piece of 320 grit in a shallow bowl containing Minwax Wipe-On Poly. After this, gently sand the damaged area. The procedure varies depending on the damaged surface. Below is how you can restore the polyurethane finish on some surfaces.

  • CabinetsImage of kitchen cabinets but How Many Coats of Primer on Cabinets

It doesn’t matter whether the cabinets are in the kitchen or bathroom. If they are exposed to cooking oils and excess moisture, they can quickly get damaged. You can consider using the Miniwax Wood Cabinet Cleaner whenever you detect grease or water spots build upon the cabinet. 

For polyurethane, use Miniwax Wipe-Only Poly. Use the wet sanding technique to keep the sandpaper loaded with Wipe-On Poly so it cant leave scratches. After that, allow the Wipe-On Poly to penetrate for approximately five minutes before removing any excess with a clean lint-free cloth.

  • Floors

Floors gradually undergo wear due to the grit on the sole of your shoes. You can reduce this by strategically placing the doormats and throw rugs. The best way is by regularly vacuuming and cleaning with Minwax Hardwood Floor Cleaner. The cleaner removes the grit before it harms the floor surface. Also, the cleaner can remove grease and stubborn dirt without leaving any residue.

  • Furniture

Most wood furniture has a layer of stain protected by two or more coats of clear protective finish like polyurethane. With time, wear and tear can leave scratches and nicks on the surfaces. You can quickly fix this using the Minwax Wood Finish Stain Markers. You will get these products in up to eight popular colors. 

If you use this product for your furniture, you rest assured of an easy way to touch up minor blemishes without resorting to costly refinishing. But if there’re deeper scratches that require filling, use the Minwax Blend-Fil Pencil. The product fills holes and scratches on your furniture while restoring the wood’s original color.

  • Rooting Wood

The worst enemy of wood is water. If given enough time, it softens and causes decay even in the toughest hardwoods. In case you discover any softened wood, consider using a screwdriver to remove the rotted wood fibers. 

It would be best to allow the area to dry and then brush on a liberal coat of Minwax Wood Hardener. Allow it to penetrate the wood grains, dry, and harden inside the wood fibers. You then fill the remaining cavity with Minwax High-Performance Wood Filler.

  • Nail Holes

It doesn’t matter whether the nail holes are in furniture or trim work. You can fill these holes easily using the Minwax Wood Putty. Applying this product is as easy as pressing a small amount into the nail hole and whipping off the excess with a soft cloth.

How Do You fix Streaky Polyurethane?

When applying polyurethane on wood surfaces, you might end up with a streaky poly. You can quickly fix this by cleaning off the surface with a damp piece of cloth. Doing this allows you to check if there is an even surface. If you realize that the poly is thick, it’s recommended to sand again. 

However, if it’s not thick, add another coat of polyurethane. When you’re done, check again to see if there is an even sheen. Suppose you get impressed with the sheen level, buff it with wax. You can also continue with higher grits for a glossier finish. To prevent streaks, use a foam brush or wipe on poly.

Why Did My Polyurethane Turn White?

Suppose you apply shellac, lacquer, or another quick-drying finish in areas with high humidity. The finish can turn milky. The reason is that the moisture trapped in the finish didn’t have enough time to evaporate before the finish hardened. 

For polyurethane, this is not the case, not even for the water-based type. Your polyurethane turned white because of the buildup of zinc oxide, commonly used as a flattening agent. Both oil and water-based polyurethane are varnishes. Therefore, they always dry to a naturally glossy finish.

Manufactures had to add the zinc oxide to act as the flattening agent to create a satin or matte finish. The zinc oxide helps in dispersing the reflected light and cutting the gloss out of the sheen. Mainly, these materials collect at the bottom of the can, and that’s why it’s always important to stir well before using.

Two Reasons Why the Finish May be Hazy

  • You may have applied too many coats of the satin finish.
  • You used the materially rich in flattening agent that had settled at the bottom of the can due to poor mixing of the varnish.

What to Do to Prevent Polyurethane from Turning White 

First, you should check whether the polyurethane has dried. Suppose you use a water-based material. It would be best if you didn’t worry because it usually appears milky before it dries. The milky color is primarily due to the emulsion that carries the resins. Therefore, if the finish is yet to dry, you have to wait until the milkiness disappears. 

In some circumstances, the finish might be completely dry to touch. So you don’t have the proper way of correcting the buildup of the flattening agent. In such cases, applying another coat can’t help remove the cloudiness as seen in lacquer since polyurethane cures.

It even becomes worse since the best remedy is to strip, sand, or scape the varnish and do it again. However, if the cloudiness is caused by moisture and isn’t widespread, the following simple steps might help.

  • Step 1 

It would help if you covered the affected area with petroleum jelly or mayonnaise. 

  • Step 2

After covering, wait till overnight to wash the wood clean. In this case, you will see an improvement since the oil in the petroleum jelly or mayonnaise helps to leach out moisture from the finish.

  • Step 3

Repeat the above procedures if you noticed a difference, but the cloudiness is still there.

Here’s how to fix the whitening of your finish:

Way to Prevent the Recurrence

Before determining how you can solve the issue with cloudiness, consider tips below on how you can prevent your polyurethane from turning white. 

  • Stir 

Ensure that you stir your varnish until the tip of the material is no longer covered with the white residue when you scrape it on the bottom of the can.

  • Allow the wood to dry completely.

Before applying polyurethane, allow the wood to completely dry. You can test for dryness with a few drops of water. If the drops don’t soak in immediately, the wood surface is too wet to paint.

  • Avoid building up coats.

Suppose it’s a must that you build up coats. Consider using a gloss material. Afterward, you can use satin for the last one or two coats. The procedure helps in cutting the sheen.

Image of wood being painted but How Long Can I Wait to Paint After Priming?What is the Best Applicator for Polyurethane?

There are different ways through which you can apply polyurethane. Using a fine bristled brush, either natural or synthetic, will leave your surface with a perfect finish. Such brushes are better for fine details and molded edges.

You can also use a foam brush since, apart from being inexpensive, they work well for most flat surfaces. To avoid the obvious brush strokes, don’t use the other types of cheap bristle brushes.

How to Apply Oil-based Polyurethane

Even though thinning oil-based polyurethanes with naphtha or mineral spirit is unnecessary, you may choose to use this method. Before doing that, ensure you check the manufacturer’s recommendations. Thinning prevents buildups while helping the finish flow into nooks and crannies and fine details. 

Use either fine bristled or a foam brush to apply the polyurethane gently. Ensure that the brush strokes are parallel to the wood grain. For the finish, use a sufficient but not overly thick coat. Also, ensure you complete each application area with long straight strokes. The strokes will help brush out as many polyurethane bubbles as possible. If there few remaining bubbles, they will typically disappear after a few moments. 

Give the first coat adequate drying time, then lightly sand the entire surface parallel to the grain. You can do the sanding using 320 grit sandpaper. Ensure that you sand carefully since polyurethane sands easily. After sanding, remove the dust with a vacuum cleaner and a tack cloth before the second coating.

Repeat the above steps until you attain the desired level of protection. I will recommend a minimum of two coats for perfect protection. You can use up to three coats for surfaces that will see hard wear or those exposed to moisture. When you are done with your final coat, you may rub out the finish with a #0000 steel wool followed by paste wax for an excellent luster. 

How to Apply Water-based Polyurethane

Image of a water based polyurethane for hardwood floor refinishingUnlike oil-based polyurethane, you need to sightly “rough up” the stained surface before applying. You’ll consider using some synthetic steel wool. Oil and water never mix. Therefore, the “rough up” process will prevent polyurethane from beading on the surface.

The application technique is similar to that of oil-based. You need to apply a thin coat with a foam pad, cloth, or brush. Remember to work with the grain and don’t apply too much to avoid raising the grain.

The first coat will take only a couple of hours to dry. Then you can apply the second one. If you correctly use these processes, you may not have to sand between coats, as seen in oil-based polyurethane. At least three coats will be fine for lightly used projects. Plan on approximately four coats on floors and other pieces that require maximum protection.

How Do You Polish the Final Coat of Polyurethane?

After you are done applying the desired amount of coats, spray on the final coat with an aerosol polyurethane. Allow it to cure over the night. You then remove any dust nibs using a 1500 grit sandpaper. You may also use a piece of a brown paper bag. The final step is to buff the finish to a high shine with an automotive paste wax with fine abrasives to polish the finish even further. You can do this using either a polishing pad or a soft cotton rag. 

Final thought

We have seen that polyurethane plays an essential role in protecting wood surfaces. Apart from offering protection, it provides a distinctive, beautiful look on the surface. You can use the item for different purposes ranging from the floor where wear is immense to the bathroom and kitchen where surfaces are constantly exposed to moisture. The application methods are straightforward, and the product is generally durable. So…

Can You Put Polyurethane Over Old Polyurethane?

Yes, you can. But ensure you use the proper steps while preparing the finish. 

As I wrap up this article, I hope you have benefited from the in-depth details concerning applying polyurethane over old polyurethane. But let not this be the end. If you have any unanswered questions, observations, or suggestions, don’t hesitate to drop them in the comment section below.

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Tyron Otieno

Tyron is an avid woodworker and writer. He founded this website to help other woodworkers, whether hobbyists or professionals by sharing his knowledge and experiencie after a decade of woodworking.