How To Fix Polyurethane Mistakes

Polyurethane protects wood surfaces incredibly well if applied correctly. The process of applying polyurethane requires many meticulous steps. If mistakes are made at any point in that process you can end with a host of problems. If you have mistakes you wish to correct in your polyurethane finish, I have methods to help.

Bubbles in Polyurethane Finish

One of the most common errors when applying polyurethane are bubbles in the finish. Bubbles can occur for a multitude of reasons. Here are a few of the most common:

  • Air trapped in the polyurethane
  • Air gaps within brush bristles 
  • Off-gassing from wood

Preventing Bubbles

To prevent bubbles as your apply poly:

  • Gently stir your polyurethane prior to applying. Never shake your can of polyurethane.
  • Ensure that you are using the right type of brush or as recommended by the polyurethane manufacturer. Generally, water-based polyurethane can be best applied with synthetic-bristles brushes while oil-based polyurethane performs better with natural bristles.
  • Avoid working in cold temperatures if possible. Construction materials tend to off gas more slowly on mild days. Avoid working in high temperatures if possible. Polyurethane will cure more slowly on cold days.

Removing Bubbles

If you notice bubbles as you are applying your polyurethane, you can pop them with the tip of your brush. If your polyurethane has already set, fixing the issue becomes a little more involved.

In order to remove the polyurethane bubbles, follow these steps:

  1. Use fine grit sandpaper to remove bubbles down to your original surface.
  2. Remove grit and dust with a damp napkin.
  3. Allow the surface to dry thoroughly.
  4. Reapply thin layers of polyurethane.

Puddling in Finish

Puddling occurs when too much polyurethane is allowed to pool in an area.

  • When working with wood floors, this can occur when surfaces are uneven.
  • With furniture, concave surfaces are often intentional within the design and function of the piece. Often times, furniture is sprayed because of the concave surfaces, but this doesn’t eliminate pooling.

More challenging than bubbles, addressing puddling can be a much more involved process.

Preventing Puddling

Properly preparing surfaces prior to applying polyurethane is the most reliable way to prevent puddling.

  • Sand floors evenly, working out all cupping and crowning.
  • When working with furniture that has contours and uneven surfaces, ensure that the entire piece has received a high-grit sanding.
Hand holding a brush to apply polyurethane to a wooden surface.

Applying polyurethane thinly and evenly is much easier on smooth surfaces. Thining your polyurethane is a very easy process.

Correcting Puddling

If you notice your polyurethane puddling as you are applying it, use your brush to thin out the puddled area. Work quickly, especially when working with water-based polyurethane. If your polyurethane is given time to start setting, you will wind up with brush-bristle marks marring your surface. Brush marks in your finish will require sanding and reapplication of polyurethane.

While not ideal, if you are unable to brush out the puddle, you can also wipe up excess polyurethane with a rag. Be sure to keep the poly-soaked rag away from any ignition sources. Polyurethane is flammable. Dispose of the rag properly.

If the puddle of polyurethane has set, use the following steps to address the issue.

  1. Sand the puddle area. Work carefully using coarse-grit sandpaper to remove the thick layer of polyurethane. As you near the original work surface, switch to fine-grit sandpaper to avoid damaging your wooden surface.
  2. Sweep and discard the fine residue from sanding.
  3. Use a cloth damp with mineral spirits or water to wipe the sanded surface clean.
  4. Allow the work area to completely dry.
  5. Apply polyurethane to the sanded area following manufacturer’s instructions.

Other Common Issues in Polyurethane Finish

There are many issues that result from expired, poorly mixed, or contaminated polyurethane. If your project exhibits some of the following qualities, you may have bad polyurethane.

  • Rough finish
  • Discoloration

Lets review how to address each of these problems.

Rough Finish

If you notice that your surfaces are rough to the touch following the manufacturer’s recommended dry time, I recommend checking for the following issues.

Improperly prepared wood

Polyurethane will take the shape of the surface which it coats. A rough-textured wood will remain rough textured after applying polyurethane.

If you want a smooth finish, be sure to take all the necessary steps to prepare your surface to receive the polyurethane. Sanding rough wood is not particularly fun, but it is better than having a rough finished piece.

Failure to sand between coats

Whether you are applying oil-based or water-based polyurethane, it is recommended to lightly sand between each coat. If you failed to sand between coats, follow these directions:

A man sanding the stare case's wooden surface
  1. Lightly sand surface with 220-grit sandpaper.
  2. Using a rag dampened with mineral spirits, wipe all loose particles created by sanding.
  3. Ensure the surface is totally dry.
  4. Apply or spray polyurethane according to the manufacturer’s instructions. Water-based polyurethane usually needs 4-6 hours of dry time between coats while oil-based needs up to 24 hours. 
  5. Apply the next layer of polyurethane.
  6. Repeat steps 1-5 as necessary.

Discoloration

As opposed to a finisher like lacquer, polyurethane does have a natural yellowing component once it dries. This is normal, but poly that has gone bad might discolor a lot more.

Discoloration may be an inherent feature of your chosen polyurethane. Oil-based polyurethane will take on deeper hues as it ages. Polyurethane can also absorb oils from the material it is coating causing discoloration. If consistently exposed to UV rays, polyurethane can yellow over time.

Prior to more aggressive fixes, first try to scrub the area with dish soap and a sponge. While this method will likely not remove all yellowing, it can remove surface contaminants and greatly improve the looks of your floor or furniture.  

To address the issue of polyurethane discoloration: 

  1. Remove all discolored polyurethane. 
  2. Prepare surface to receive new polyurethane by sanding with increasingly finer grit sandpaper until you’ve achieved the desired surface.
  3. Apply new polyurethane according to the manufacturer’s instructions.
  4. Sand gently between each coat.
  5. Wipe away all fine particles with a rag and mineral spirits.
  6. Allow the surface to dry.
  7. Apply next coat of polyurethane.
  8. Steps 3-7 can be repeated as needed.

Tip: If you are removing polyurethane from a large, flat area such as a floor, you can use a floor sander or drum sander to speed up the process. These tools can be rented and usually include a short tutorial on how to safely and properly use them. For smaller projects, palm sanders, mouse sanders, or random orbital sanders are better choices.


Alternatively, if you would prefer to avoid the mess created by sanding discolored polyurethane, consider using chemical stripping gel and a scraper to remove discolored polyurethane. 

Supplies

You will only need a few tools to correct polyurethane mistakes.

  • Sand paper
  • Brush
  • High quality polyurethane
  • Safety gear (safety glasses and respirator)

A few tips to achieve the best results:

  • Do not expose your polyurethane can to extreme high or low temperatures.
  • Clean surfaces well before applying polyurethane.
  • Follow manufacturer guidelines when applying polyurethane.

Type of Polyurethane

There are two common types of polyurethane. Oil-based and water-based polyurethane can both provide a lasting barrier to protect your wood floors or furniture when applied. All of the above methods I listed correct problems in both water and oil poly.

Oil-based polyurethane is thicker, requiring fewer coats. Oil-based polyurethane also tends to darken wood over time in a process called ambering. It has a longer drying time and emits much higher levels of VOCs (volatile organic compounds) that can irritate lungs, cause headaches and nausea. 

Water-based polyurethane is thinner, dries more quickly and clearly, and has fewer VOCs. Water based polyurethane mimics polyacrylic in many ways.

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Conclusion

Polyurethane is a highly durable wood finish that can protect your wooden project for years if applied correctly. However, there are many steps in correctly applying polyurethane. Ensure that your project is well sanded and cleaned before beginning. Apply polyurethane in several thin layers. Between each layer, sand and clean surfaces. Nearly all polyurethane errors can be addressed with materials as cheap and simple as sandpaper, mineral spirits, and quality polyurethane.

An expert at home repair, remodel, and DIY projects for nearly 40 years. His first experience came in completely restoring an antique home. Completely redone from the inside out, and restored to its original form, the home is a featured design by renowned Southern California Architect Cliff May, considered to be the father of the California Ranch Home. Now Dennis spends his time on fine woodworking projects and tool comparisons.