Woodworking enthusiasts and professionals have long pondered the question: will polyurethane keep wood from cracking? At first glance, applying a protective finish like polyurethane should seal the wood and prevent cracks from forming. However, wood is a complex, natural material that responds dynamically to its environment. To understand polyurethane’s cracking prevention abilities, we must dive deeper into the causes of wood cracks and how polyurethane interacts with wood. Grab a fresh cup of coffee, get comfortable, and let’s unravel this wood finishing mystery together!
A Primer on Polyurethane
Before analyzing polyurethane’s crack-stopping powers, we first need to understand what polyurethane is and why it’s commonly used as a wood finish.
Polyurethane is a polymer resin created through a chemical reaction between a polyol resin and an isocyanate. The characteristics of the resulting polymer depend on the original components used to manufacture it. Polyurethane is valued as a wood finish because it:
- Provides a smooth, durable, and water-resistant coating
- Offers good scratch and abrasion resistance
- Produces a surface that is easy to clean
- Comes in a variety of sheens from matte to high gloss
- Has little odor compared to many oil-based finishes
Polyurethane can be formulated as either an oil-based or water-based product. Oil-based versions are more durable and water-resistant but have longer drying times and stronger odors. Water-based polyurethanes dry faster, have low VOCs, and are easier to apply and clean up after use.
Now that we’ve got a handle on what polyurethane is, let’s move on to understanding why wood cracks in the first place.
The Science Behind Wood Cracks
Wood seems solid, but it constantly moves and shifts in response to moisture in the surrounding environment. This moisture-driven movement is the primary cause of cracks in wood products.
The main factors that contribute to wood cracking are:
- Wood anatomy – Wood’s natural grain structure and growth rings make it weaker in some dimensions. Cracks often follow the wood grain.
- Moisture content – Wood swells and shrinks as it gains and loses moisture. These dimensional changes produce internal stresses.
- Relative humidity – Rapid changes in humidity cause the wood moisture content to fluctuate, resulting in expansion and contraction.
- Temperature – Heat causes wood to lose moisture, while cold causes it to absorb moisture from the air. Frequent temperature swings stress the wood.
- Wind and sun – Exposure to wind and sun can rapidly remove moisture from wood surfaces leading to cracks.
- Wood movement – Wood species expand and contract at different rates, resulting in joint cracks.
- Density – The denser the wood, the less it expands and contracts with moisture changes. Softwoods tend to crack more than hardwoods.
In addition to moisture-related cracks, wood can also crack from physical damage, incorrect woodworking techniques, poor storage and handling, or the natural aging process. But for our purposes, moisture-induced cracks are the most relevant when discussing polyurethane finishes.
Will Polyurethane Prevent Cracks?
Now that we understand why wood cracks, we can dive into the question: will polyurethane prevent cracks from forming? Unfortunately, there isn’t a simple yes or no answer. Polyurethane can reduce cracking but is not a foolproof preventative measure. Let’s analyze the pros and cons of using polyurethane to limit cracks.
- Forms a moisture barrier – Polyurethane seals the wood surface from direct contact with humidity in the air. This helps moderate moisture absorption and dimensional changes.
- Provides a stiff surface film – The hardened film of polyurethane mechanically strengthens the wood surface to resist cracking.
- Blocks UV radiation – Polyurethane prevents the sun’s UV rays from rapidly evaporating moisture from the wood surface.
- Enhances dimensional stability – Sealing the wood slows moisture exchange between the wood surface layers and the environment.
- Offers abrasion resistance – The tough polyurethane film prevents the wood surface from becoming roughened or damaged, reducing cracks.
- Cannot eliminate wood movement – While polyurethane slows moisture exchange, wood will continue to absorb and release moisture from the sides and ends that are not coated.
- Surface cracks can still occur – If the wood swells enough, it can overpower the polyurethane film and cause surface cracks.
- Problems at joints – Joints are vulnerable points where different wood pieces meet. Polyurethane cannot prevent cracks from wood movement at joints.
- Existing cracks reappear – Any cracks already present before the polyurethane is applied can reappear later since the wood still moves underneath the finish.
- Brittle over time – As polyurethane ages, it becomes more rigid and brittle. This makes it more prone to cracking if the wood underneath shifts.
So in summary, polyurethane provides some cracking protection but does not completely restrain wood movement. It is most effective for reducing surface cracks but cannot fully prevent cracks at joints or from excessive dimensional changes in the wood itself.
Application Tips to Limit Cracks
While polyurethane alone cannot guarantee crack-free wood, proper application techniques can maximize its protective abilities. Here are some best practices for using polyurethane to reduce cracking:
- Use an oil-based polyurethane – The oil-based versions are better at sealing the wood surface from moisture. Be sure to allow for the longer dry time.
- Apply multiple coats – Building up a thicker film provides better moisture protection. 3-5 coats are recommended for crack prevention.
- Sand lightly between coats improves layer adhesion for a stronger moisture barrier.
- Apply even coats – Uneven application can lead to localized cracking if the moisture protection is inadequate in spots.
- Seal all surfaces – For maximum protection, polyurethane should be applied to all wood sides, not just the top.
- Maintain stable conditions – Controlling relative humidity and temperature helps minimize wood movement both before and after the polyurethane is applied.
- Use proper joint design – Joints that allow for wood movement, like tongue and groove, will better resist cracking. Avoid gluing boards directly across the grain.
- Apply finish to bare wood – Polyurethane adheres better to raw wood than other finishes. Avoid applying it over existing stains or surface coats.
Proper surface preparation, application techniques, and wood acclimatization are all crucial for getting the most protection from polyurethane against cracks. Taking shortcuts increases the risk of cracks forming.
Alternative Crack Prevention Options
Polyurethane is not the only weapon that woodworkers and wood product manufacturers can use to prevent cracks. Here are a few other strategies:
Chemical stabilizers reduce wood shrinkage and swelling by reacting with hydroxyl groups in the cell walls. Common stabilizers include polyethylene glycol (PEG), methyl methacrylates, and aldehydes. Stabilizers are applied through spraying, brushing, or vacuum pressure treatment.
Maintaining a stable relative humidity between 45-55% can minimize dimensional changes in wood products after manufacturing. This may involve equipment like dehumidifiers, humidifiers, and dedicated humidity-regulated spaces.
Choosing woods that are naturally more dimensionally stable, like quartersawn lumber, reduces cracks. Quartersawn boards move less across their width compared to flat sawn boards. Dense, old-growth woods are also more stable.
Adding wood battens, splines, butterfly keys, bowties, or epoxy repairs helps reinforce weak areas and prevents existing cracks from propagating further through the wood.
Regular inspection, cleaning, minor repairs, and refinishing can catch issues early before major cracks develop. Addressing problems promptly limits damage.
Penetrating oils like tung and linseed offer moderate moisture protection with less surface film buildup than polyurethane. They maintain some water resistance while allowing the wood to breathe.
Each strategy has pros and cons, but often a combination of approaches is the best recipe for success. Wood stabilization, polyurethane finishing, and humidity control could offer a one-two-three punch against cracks!
Frequently Asked Questions
Q: Will polyurethane fill existing cracks in wood?
A: No, polyurethane is not a crack filler. It only seals the surface and prevents additional moisture from penetrating cracks. The cracks should be filled with a flexible wood filler before applying polyurethane.
Q: Can I apply water-based polyurethane over an oil-based stain?
A: Yes, water-based polyurethane is compatible with oil-based stains as long as sufficient drying time for the stain is allowed.
Q: How long does polyurethane take to fully cure?
A: Oil-based polyurethane cures slower than water-based. Allow at least 24 hours between coats and 30 days for full curing before subjecting the surface to heavy use or moisture.
Q: Does UV radiation from the sun damage polyurethane finishes?
A: Yes, UV exposure will gradually degrade polyurethane. Reapplication every 2-5 years is recommended for surfaces exposed to strong sunlight.
Q: Can I use polyurethane over wood that has already started cracking?
A: Applying polyurethane can slow the spread of existing cracks but it works best on new wood that does not have pre-existing damage. Limiting wood movement through moisture control is key for stopping cracks in distressed wood.
Q: How do I prevent blotching when applying polyurethane?
A: Wiping on thin coats, sanding between applications, using wood conditioner first, and applying a stain controller can all help minimize blotching from uneven finish absorption.
Preventing Cracks in Specific Wood Products
The principles we’ve covered generally apply when using polyurethane to reduce cracking for various wood products. Here are some tips tailored for specific items:
- Seal all surfaces – Applying polyurethane to table undersides, chair rungs, and backs prevents moisture absorption.
- Maintain indoor humidity – Keep relative humidity between 45-55% year-round.
- Adjustable glides allow movement and prevent stresses between furniture and the floor.
- Avoid large single-piece tabletops – Opt for properly joined boards instead.
- Use stable materials like quartersawn oak, teak, and mahogany.
- Only install over suitable subfloors – Ones that resist moisture like concrete, plywood, or OSB.
- Leave 1/4″ expansion gap along walls – Allows space for natural floor movement.
- Use hardwood species rated for flooring – Oak, maple, ash, and hickory hold up well.
- Stagger end joints – Avoid aligning end joints which can lead to peak seasonal gapping.
- Maintain indoor humidity between 35-55%. Higher for solid wood floors.
- Allow logs time to settle before chinking – This prevents cracks from log movement later.
- Use squared logs with interlocking corners – Provides better stability than round logs.
- Ensure proper foundation drainage – Prevent moisture buildup under home.
- Apply polyurethane evenly to log interior and exterior.
- Use elastic chinking materials like urethane.
Wood Bowls & Platters
- Turn wood from the center of the log – This area experiences less shrinkage.
- Seal endgrain twice as thick – Endgrain absorbs more finish and moisture.
- Allow newly turned bowls time to stabilize – At least 6 weeks.
- Maintain consistent shop humidity while drying.
- Reinforce cracks with inlaid splines after turning.
Evaluate your wood project and environmental factors to select appropriate finishing methods for preventing cracks. Don’t rely on polyurethane alone – smart wood selection, construction techniques, and moisture control are also critical.
We’ve dug deep into whether polyurethane can prevent cracks in wood objects and determined that it is no silver bullet. Polyurethane provides important moisture protection but has its limitations. Cracks can still occur from inherent wood stresses, joint movement, extreme conditions, and as the finish deteriorates over time.
You can minimise cracking issues by understanding how wood cracks, applying polyurethane properly, controlling humidity, selecting stable wood, and incorporating structural reinforcements. It takes a comprehensive approach to keep wood stable and durable long-term. Ultimately there are no guarantees when working with natural wood products, but with careful design and finishing, you can prevent many headaches.
So while polyurethane alone won’t completely stop cracks, as part of a broader stabilization and protection strategy, it remains a versatile, easy-to-use finish that can help wood objects stay smooth, beautiful, and crack-free for years of enjoyment. Just don’t expect miracles from the polyurethane alone!
Now that we’ve settled the debate over polyurethane’s powers to prevent wood cracking, it’s time to head to the workshop with greater knowledge. Let’s put this information into practice by finishing some projects with confidence. With science on our side, we can work with wood’s natural properties to create lasting items free from distressing cracks. It’s time to break out the polyurethane and brushes – let’s get finishing!