Film-Forming vs Penetrating Oil Wood Finishes

By Steve Getsiv, 06/10/19

Are you looking for a new wood stain that won't cause your exterior wood furniture to chip or peel? Which types of finishes work? Which don't? The two most common wood finishes are film-forming and penetrating oil finishes. Although the finishes may look somewhat similar at first on wood, they differ vastly in breathability, longevity, and upkeep.

What is a Film-Forming Finish?

A film-forming finish creates a coating or "film" on top of the wood surface. This coating helps protect the wood by reducing the effects of weathering. These finishes can be painted or sprayed on and need to be stripped and re-applied at the first sign of bubbling or cracking. Film-forming finishes are commonly used on interior furniture and cabinetry, away from the elements.

Potential Drawbacks to film-forming Finishes

There are drawbacks to using film-forming finishes, especially when not protected from the outside environment. Since the finish does not penetrate the wood, the natural occurrence of expansion and contraction in wood with changes in humidity will create cracks in the film, eventually chipping and peeling. The cracks and peeled finish expose the unprotected wood to moisture and sunlight, causing advanced deterioration and more peeling and chipping near the initial finish failure. To repair the finish, you need to strip the finish off by sanding, or by using toxic chemicals that need to be handled with care – both should be used with a mask/breathing apparatus.

Although a recently applied film finish can make a deck, log home, or wood furniture look exquisite, this feeling of bliss is short-lived. Water penetrating through cracks will become trapped between the un-breathable finish and the wood. As it sits there and pools on top and eventually within the wood, it accelerates the decay process. In no time you can have areas of rotting boards underneath your premium finish. Owners of outdoor furniture need to take extra precaution to keep moisture exposure to a bare minimum.

What about Marine Spar Varnish?

Sailors needed something to protect their boat while allowing for the natural flexing of wood being exposed to sun, salt, and water. Oil was added to a solvent and resin to create a flexible varnish that could expand and contract.

Potential Drawbacks of Marine Spar Varnish

Although developed with the aquatic life in mind, marine spar varnishes only create a barrier between the wood and the water. Most marine spar varnishes on the market today act as a "Saran Wrap", encapsulating the wood itself; and they need to be frequently re-applied to prevent failure. There are many application downfalls as well. Common application problems are: slow drying due to too thick of a layer being applied; blistering, from applying the product directly on too hot of a surface or sun exposure immediately after application; fish-eye effect, from surface debris caused by contaminants, improper sanding or applying over an unwashed surface. However, marine spar varnishes can be a useful tool in aquatic applications if selected, applied, and maintained properly – it's A LOT of work – just ask anyone who owns a boat!

Polyurethane Finishes

Polyurethane finishes are film-forming finishes which are typically used on cabinets and furniture and can be applied by brush or sprayed. Inside the house, these types of finishes can last longer because there is not a high variability of moisture or humidity and therefore the wood underneath the finish is stable. Polyurethane finishes should never be used on exterior decks, wood siding, or exterior furniture.

What is Penetrating Wood Stain?

Penetrating oil wood stain is a type of finish that absorbs into the wood and does not fail as often, or as quickly as film-forming finishes which do not allow the wood to move. This type of stain is typically the best for exterior furniture because it does not peel or flake off over time. Even a low-quality penetrating oil will simply grey out or weather over time. Care is needed since over-applying a penetrating oil can turn it into a film-forming finish that has the potential to peel or flake off over time.

How to Use Penetrating Wood Stain After Applying a Film-Forming Finish

In order to apply penetrating oil over a previously applied film-forming finish, the old finish will need to be completely removed. Once cleaned down to bare wood, then the new penetrating oil wood stain can be applied.

Premium Quality Pure Tung Oil Stain

Nova USA Wood Products provides a premium, penetrating oil stain that is perfect for hardwoods and softwood in applications such as decks, siding, outdoor furniture and even fencing. Our ExoShield premium wood stain is the highest quality wood stain ever produced. ExoShield provides superior protection for all exterior wood applications. In decking applications our product is guaranteed to look good for one year, and instead of getting the whole deck sanded and pressure washed for another finish, you simply recoat it with our ExoShield stain. Siding applicaitons require maintenance every 3 to 5 years.

By Steve Getsiv, 06/10/19

"I’m a chainsaw carver in Oregon. On a quest to find the perfect finish for my outdoor art. Any strides relating to that see appreciated"

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