What is the downside of oil based polyurethane?

Polyurethane is a popular finish and sealant used on wood floors and furniture. It provides a durable protective coating and comes in two main types: oil-based and water-based. Both have their advantages, but oil-based polyurethane also has some drawbacks that are important to consider before using it. This article will provide an overview of what oil-based polyurethane is, its benefits, and most significantly, its downsides. Understanding the pros and cons can help make an informed decision when choosing a finish for wood projects.

What is Oil-Based Polyurethane?

Oil-based polyurethane is made up of natural oils, resins, and solvents. The oils are typically either tung oil or linseed oil. These provide water resistance and flexibility. Resins such as alkyds, give the finish its tough, durable properties. Solvents which are mineral spirits derived from petroleum, act as a carrier or thinner.

Unlike water-based polyurethane, oil-based contains little to no water. It cures through a chemical reaction when the solvents evaporate. The high oil content is what makes oil-based poly so glossy, smooth, and durable. However, the chemical makeup also leads to its downsides.

Benefits of Oil-Based Polyurethane

Before examining the disadvantages, it’s important to understand why oil-based polyurethane is still commonly used. Here are some of its advantages:

• Extremely durable and long-lasting finish
• Provides excellent moisture resistance
• Has high resistance to scratches, scuffs, and abrasion
• Leaves a smooth, glass-like surface
• Enhances and deepens the natural wood grain
• Dries to a harder film than water-based
• Less prone to leaving brush marks

For surfaces that need maximum protection and the highest quality finish, oil-based polyurethane is still the product of choice for many woodworkers. Now let’s look at the downsides that also need careful consideration.

Long Drying Time

One of the biggest disadvantages of oil-based polyurethane is its extremely lengthy drying time. While water-based varieties can dry within 1-2 hours, oil-based polyurethane needs up to 24 hours between coats. The reason it takes so long to dry is because of the slow solvent evaporation process.

Each coat must be allowed to fully cure before the next coat can be applied. Rushing this drying time often leads to problems like bubbling, cracking, or an uneven finish. For large surfaces, drying can take days or even weeks before it’s ready for light use. The long dry times make the project take significantly longer compared to water-based options.

Typical Drying Times for Oil-Based Polyurethane

Coat	Dry to Touch	Dry to Recoat	Full Cure
First	6-8 hours	24 hours	1 week
Second	8-10 hours	24 hours	1 week
Third	8-10 hours	24 hours	1 week

Strong Solvent Odor

The petroleum-based solvents that allow oil-based polyurethane to flow and penetrate into the wood also create an extremely strong smell. The odor is similar to paint thinner and can be quite unpleasant and overwhelming.

This means proper ventilation is critical when applying oil-based poly, otherwise the fumes can build up quickly. Working outdoors or in areas with open windows is ideal. Even when dry, the smell can linger for days or weeks after application. Those sensitive to chemical odors may want to opt for a water-based product instead.

Cleaning Difficulties

Clean up is far more challenging with oil-based poly since it can’t be cleaned up with just soap and water. The oil coatings require mineral spirits or paint thinner to dissolve the finish and clean brushes and equipment. This can be inconvenient, especially if proper solvents are not on hand.

There are also more safety precautions needed when handling the chemical cleaners. Proper disposal of used solvents is required and offers yet another hassle compared to simply washing water-based poly tools.

Environmental and Safety Concerns

The high VOC (volatile organic compound) content in oil-based poly means it can be potentially harmful to health and the environment. Prolonged exposure to polyurethane fumes poses risks like dizziness, headaches, and irritation to lungs, eyes, and skin. There are also risks when handling solvents like mineral spirits.

Spills or improper disposal of oil-based products can contaminate groundwater and soils. While taking proper precautions minimizes risks, water-based polys are overall more user and environmentally-friendly.

Not Ideal for Outdoor Use

Although oil-based polyurethane holds up exceptionally well indoors, it can deteriorate more rapidly when exposed to sunlight and moisture over prolonged periods. The UV rays tend to cause fading, yellowing, and loss of glossiness in the finish faster than with water-based products.

It may also experience cracking or peeling outdoors over time. Therefore, oil-based polyurethane is not necessarily the best choice for items like outdoor furniture that need weather-resistance.

Prone to Ambering Over Time

Ambering refers to the gradual development of a yellow/orange tint in the finish. All oil-based polys are susceptible to this effect since the natural oil resins cause a color change when exposed to light. The amber tint develops slowly, starting clear when first applied. This color shift can alter the look of light colored woods.

While newer formulations have improved, ambering potential should still be considered especially if aiming to maintain a pristine clear finish on light woods. Using water-based instead avoids this aging effect.

Difficult to Patch or Repair

Fixing damaged spots in an existing oil-based poly layer can be tricky. Since it cures chemically, new poly can’t bond well to already dried layers. This means damaged areas must be carefully sanded and stripped before fresh coats can be reapplied. Lots of patience and expertise is required to blend patch jobs seamlessly.

With water-based poly, it’s much simpler to spot repair affected areas since the polymers re-emulsify, allowing coats to bond together. Oil-based poly is therefore less forgiving when it comes to repairs.

Higher Cost

Generally, oil-based polyurethanes cost noticeably more than water-based versions. The higher materials expenses come from the complex blend of natural oils, resins, and chemical solvents. In some cases, oil-based can be twice the price of comparable water-based polys.

The investment may be worthwhile for furniture or floors needing ultimate durability. But for more casual projects, the added cost should be taken into account when selecting a finish.

Conclusion

Oil-based polyurethanes produce an extremely attractive, protective finish that brings out the richness of wood grain. However, drawbacks like lengthy dry times, strong odors, and repair difficulties make them less ideal for certain situations. Weighing the pros and cons allows for an informed decision when choosing between oil or water-based poly.

For quick, convenient application on smaller projects, water-based is generally the easier option. But when only the smoothest, hardest finish will do for important surfaces like floors or tables, oil-based polyurethane remains a top choice despite the downsides. Understanding application tips and risks is key to getting great results.

With the right expectations going in, oil-based poly can provide a beautiful standout finish. However, the disadvantages should not be underestimated. Careful consideration of the intended use, working environment, project timeline, and safety factors will ensure an appropriate finish is chosen to highlight wooden surfaces while avoiding potential headaches.

Frequently Asked Questions

Is oil based polyurethane better than water based?

Oil based poly provides a harder, more durable finish but has much longer dry times. Water based dries faster but gives a slightly softer finish. For hardwood flooring and tabletops needing maximum toughness, oil-based is better. For simpler projects where fast drying is preferred, water-based works well.

Does oil based polyurethane yellow over time?

Yes, all oil-based polys experience some level of yellowing called “ambering”. The oils react with oxygen and UV light, causing the clear finish to take on a warm, amber tone over months or years. Newer formulations have minimized this effect.

Can you apply water based polyurethane over oil based?

No, water-based poly will not bond to an existing oil-based finish. The surface must be lightly sanded and cleaned to remove oils before successfully applying water-based poly over an oil-based layer.

Is oil based polyurethane toxic?

Oil-based polys contain VOCs (volatile organic compounds) from chemical solvents that pose certain health risks. Proper ventilation is critical to avoid dangerous exposure to fumes. Once fully cured, cured finishes are inert but care should be taken during application.

How long does oil based polyurethane take to cure?

Full curing takes up 7-30 days. Initial drying between coats is 24 hours. Rushing the curing process can ruin the finish. Adequate drying time is essential for oil-based poly to achieve maximum hardness and durability.

Key Takeaways

– Oil-based poly dries slower than water-based with 24 hours needed between coats. Total curing time can take weeks.
– Strong chemical odors require good ventilation during application.
– More difficult to clean up and dispose of solvents properly.
– Prone to yellowing and fogging of the finish over time.
– Not ideal for outdoor use or repair jobs.
– Provides an extremely smooth, glossy and durable finish.
– The preferred choice when maximum protection is needed despite disadvantages.