What Happens If You Don’t Sand Between Coats of Polyurethane? Easy Tips to Applying Polyurethane

Sanding is an integral part of finishing in woodworking, different DIYers, and woodworking enthusiasts sand for various reasons. It could be to make the surface smooth or to scuff it for projects such as staining. So what happens if you don’t sand between coats of polyurethane? 

Failing to sand between coats of polyurethane does not have a significant impact on the finish. Even so, I still advise that you sand between the coats when applying polyurethane as this will help increase the adhesion between the layers to give you a more level finish.

Sometimes you are working on a short timeline and may deem sanding as time-consuming, in that case, ensure that you apply very thin layers of polyurethane formula to your surface, all it to dry before applying the next. If you are working with water-based poly, then give it at least 6 hours to dry before applying the next coat.

In oil-based polyurethane, you will have to wait at least 24 hours of drying time before you can add the subsequent layer. Apply 2 or 3 coats of polyurethane to your surface.

Can You Add Too Many Coats of Polyurethane to Your Floors?

Well, you can add many more coats of polyurethane to your surface. However, with my vast experience in the woodworking industry, adding more polyurethane coats comes with more disadvantages than advantages.

• Generally, having more than 3 layers of polyurethane is “more.” It does not change anything other than being costly. Applying more coats will mean that you incur extra costs in acquiring large quantities of the formula.
• Additional layers of polyurethane would mean that you sand each layer, which can be time-consuming mainly when you apply more than 3 coats, and you are forced to wait so that they all dry.
• Another bad thing about applying many coats is that poly tends to get thick and gloppy and, hence, would likely peel without long.
• Oxidation is relatively high when you apply several layers of polyurethane. Therefore, you will see your surface starting to yellow faster than usual, mostly if you use oil-based polyurethane.

Usually, the application of two polyurethane layers would get the job done mostly if you are working on a low budget. For the best results possible on your surfaces, try and apply 3 coats.

Can You Apply Polyurethane Over Stain?

Yes, you can apply polyurethane over the stain. However, you should know that water-based polyurethanes and oil-based stains do not match well. Therefore, if you are looking to apply water-based poly formula over the stain, you should ensure that you sand the surface before you can apply your finish. You can choose to use wood sanders with rough sandpaper to etch the surface. Alternatively, you can use steel wool.

Types of Polyurethane Finishes

Most, if not all, poly finishes contain no highly noxious chemical urethane; however, they are made using urethane polymers like polyethers and polyesters. Additionally, these formulas contain chemical solvents as well as curing agents. Polyurethane finishes are great when it comes to protecting different wooden surfaces such as wooden floors and other wooden furniture.

You can classify different polyurethane finishes based on factors such as:

• The Sheen

You will find polyurethane finishes in up to three types of sheen. First is the High Gloss, which is usually used for exterior/interior surfaces and has a tough, flexible, top-performing, and two-component finish with a shiny surface when dry. This formula is excellent in areas that see heavy-duty impacts such as foot traffic. This formula is most common in industrial environments and is excellent at covering prepared metal like steel, iron, and other surfaces such as walls, concrete, etc.

Semi-gloss polyurethane formula when dry, gives a sheen that lies between gloss and flat. This polyurethane type is mostly used on surfaces such as furniture, doors, tables, floors, and kitchen tables. You can choose to apply it using bristle, foam brushes, and lamb’s wool applicator. Satin polyurethane, are in most cases fast-drying, they give a durable matte finish that has both shine and a little extra color.

• Application

The application method is another factor that you can use when classifying different polyurethane finishes. Many people think wipe-on as a method of applying polyurethane varnish is the best if you are looking for a clear, thin coat.

Having worked with polyurethane formulas for over 15 years, I prefer the brush-on method with less costly and disposable foam brushes or bristles. These brushes make the application simple with the best results possible, and above all, you can dispose of them after you finish your projects hence saving you the time for cleaning.

• Water-Based

Water-based polyurethane does not last as much as its oil-based counterparts do. However, most people prefer them because they are odorless and do not require you to have a lot of ventilation. These formulas contain no volatile organic compounds.

Another fascinating things about water-based polyurethane formulas are that they are fast-drying, which allows you to finish your projects in a short time. These formulas are more expensive than oil-based ones.

• Oil-Based or Solvent-Based

A bad smell characterizes these polyurethane formulas, and this is because the chemicals found in oil or solvent-based finish contain volatile organic compounds, they require that you apply them in rooms with excellent ventilation. 

Usually, these formulas last longer than their water-based counterparts. They are excellent when it comes to resisting elements such as heat, water, and other possible chemical spillages.

When first applied, this formula dries into a clear finish. However, as the poly ages, they start to turn yellow, and this is due to oxidation that occurs.

How to Apply Polyurethane

Application of polyurethane formula is a straightforward process; however, you still need to be extra careful to avoid the formation of polyurethane bubbles when the formula dries.

Here are the steps involved:

Materials Needed

Some of the materials needed for this task included the following:

• Shop vacuum for dust collection
• Sandpaper if you are working on small surfaces
• Random orbital sander for large surfaces such as hardwood floors
• Mineral spirit
• Polyurethane formula
• Applicator, brush or a roller
• Cleaning detergents

Step One: Surface Preparation

The first thing you should do is prepare your surface for the application of a polyurethane formula. Some of the things you do in the preparation include cleaning the surface, especially if your wooden structure is not new.

Sweep all the debris on the surface and wash it with clean water. You can choose to use a pressure washer provided that you regulate the pressure to avoid potential damage to the structure.

Use a cleaning detergent to scrub all the possible imperfections present on the surface; such imperfection could include stains, grease, pencil marks, etc.

Once you have cleaned the surface, allow it at least 24 hours to dry, depending on the prevailing atmospheric conditions.

You can even carry out the sprinkler test to confirm whether or not the structure has dried. In the sprinkler test, you sprinkle a few drops of water over your cleaned surface. If the wood absorbs the water immediately, it is dry; if water beads up, allow the surface extra days to dry.

Step Two: Sanding the Surface

If you are interested in ending up with a smooth finish, you should sand your surface. In case you didn’t know, your finish will be as smooth as your surface. Sandpaper with a higher grit number makes deep scratches, which might further complicate the whole project.

Therefore, ensure that you use papers with lower grits, start with a 100-grit and follow it with a finer paper at 150-grit. For extra-fine surfaces, go for 220-grit. Use the shop vac for dust collection to clean all the sanding dust to give you a clean working environment.

After you have removed all the dust particles, wipe the surface using a clean, lint-free rug that you have moistened with mineral spirits. So that you are sure the surface is dust-free use a tack cloth to wipe it.

Step Three: Stain the Surface (If Necessary)

If you intended to have your surface stained before the application of the polyurethane formula, then this is where you should do the staining.

You can choose to use a water-based or oil-based stain. If you apply an oil-based polyurethane, it would be best if you apply an oil-based stain.

If you choose a water-based stain, ensure that you do a fine sanding using 220-grit sandpaper once the stain dries. Some formulas are both stains and sealers, and you might not necessarily need to seal the surface before applying the topcoat.

In case your stain does not seal, you can do that by applying a polyurethane formula but ensure that you thin it using mineral spirits. 

Step Four: Apply Two Undiluted Coats of Polyurethane

Apply two coats of undiluted polyurethane formula to your surface. It is a good practice to apply thin layers. If you are working with water-based polyurethane, you will allow it up to 6 hours of drying time, after which you should sand the surface slightly and apply the next layer.

In case you are using oil-based polyurethane, please give it up to 24 hours of drying time before sanding and applying the subsequent coat.

For surfaces such as the floor, you can apply up to 3 layers. Choose the method that best works for you as you can brush, roll, or wipe.

Benefits of Polyurethane

If you want to talk about a finish’s versatility on your different structures, then talk about polyurethane. It has unique mechanical properties that you can manipulate and separate via polymeric creativity, making it second to none when it comes to performance.

This product has several benefits that you might not explore to completion. However, below are some of the key benefits that you need to know going forward:

• Polyurethane Fills the Gap Between Rubber and Plastic

Polyurethane bridges the two, rubber and plastic. It can withstand more loads compared to rubber, and this is because poly products are harder. Additionally, these formulas give you extra flexibility to what plastics offer. This flexibility explains why the formula resists impacts and why it is a favorite of many when it comes to finishing surfaces that see massive traffic such as the garage floor.

• The Formula Resists Abrasion

Garages experience heavy traffic, massive impacts such as metals, and other forms of movement. If you wonder why polyurethanes are great for garage usage, these formulas can resist abrasion and other forms of impact.

There are no other forms of elastomers, metals, and plastics that can give you a better abrasion resistance than polyurethanes. Its ability to resist abrasion is rated 10 times better than what other materials offer.

• Oil and Chemical Resistance

In garages and even kitchens, there’s always a chance that chemicals will spill on the floor or even on tables. Therefore, you will find most garage floors, kitchen tables, and other surfaces finished using a polyurethane formula. It is because of its strong resistance to chemicals that such surfaces are coated using polyurethane.

• Polyurethane Are Highly Resilient

The resilience of a product refers to its hardness. Suppose you are looking for formulas that provide an excellent level of resilience when applied to a surface the goes for polyurethanes. They can withstand and absorb shock. Additionally, polyurethanes are not affected by vibrations since they boast of very high vibration frequency and recover quickly in case of such impacts.

Watch on the video below how to sand between poly coats:

Conclusion

Polyurethane is a versatile compound that you can apply to many different surfaces. They add shine, beauty as well as protection to the surface over which you apply them. Even so, ensure that you use the right application methods to get the best out of your finish. With that said…

What Happens If You Don’t Sand Between Coats of Polyurethane?

At this point, you must be fully aware that nothing serious happens to your finish if you fail to sand between the coats of polyurethane. However, it remains a good practice to ensure that you get a perfectly level finish.

Also, it is essential, especially when applying water-based polyurethane to an oil-based stain. In such a case, sanding is key in improving adhesion, considering that water-based poly and oil-based stains are not a perfect match.

If you have read this post, I hope you found it helpful and that your questions regarding sanding between coats are answered. In case you have a question, opinion, or suggestion, kindly feel free to share it with me in the comment section below!