Exterior wood structures are exposed to direct sunlight (UVs), come in direct contact with water, and experience higher temperatures and humidity. Naturally, they require extra protection against decay and damage. Outdoor polyurethane is the go-to option here that keeps outer wood surfaces protected while giving an impressive finish. The following is a complete guide to provide you insights on the selection and use of polyurethane to preserve exterior woods.
What is Outdoor Polyurethane?
Outdoor polyurethane is a particular form of varnish representing a correct combination of resin and oils, quite different from regular polyurethane. It has a distinctive microscopic formulation containing chains of resin molecules. The molecules bond together too tightly when the substance dries, creating a very tough and durable finish on exterior wood surfaces.
It is more resilient than regular polyurethane and many other traditional varnishes against water, solvents, and abrasions. The most critical thing exterior wood polys do is prevent photo-oxidation in wood surfaces (decay of wood’s lignin) to protect the wood.
Polyurethane for Outdoor Use – Top Selections
While researching the effectiveness of outdoor polyurethane in exterior wood protection, we discovered a strange thing! People are more interested to know which polyurethane is the most effective than asking whether polyurethane is truly useful! So, we consider letting them know what we’ve found to satisfy their thirst! As we have our own product research method, it was no way tough for us, and what follows is the top selected polyurethane for outdoor wooden surfaces.
Best Overall – Minwax Helmsman Spar Urethane
Best Oil-Based – Rust-Oleum Ultimate Spar Urethane
Best Self-Leveling & Fast Drying – ZAR Ultra Fast Exterior Polyurethane
Best Spray – Deft Defthane Interior Exterior Aerosol
Best Marine – TotalBoat Gleam Spar Varnish Polyurethane
Buying Exterior Polyurethane – Considerable Facts
Producers often make different types of polyurethane to suit the nature of the woods. So, the first step in choosing outdoor polyurethane is to know about the species and nature of your wood. Start considering the following facts after that simple step.
Generally, polyurethane comes in two different forms – waterborne and oil-based. Water type is more susceptible to sunlight and exterior environment. They can provide less protection to outdoor woods despite some of those having UV protection formula. On the other hand, oil-based polys are more resilient against outside adversaries, in addition to UV blocking capacity. So, go for the oil-generated options with the “for outdoor use” label from the manufacturer.
As long as you’re planning to take no help from a professional, it is always wise to choose an exterior wood finish that is easy to apply.
It is one of the most critical features required for a good exterior wood poly. UV blocking prevents the damaging sunlight from getting through the woods. It is necessary for all outdoor wood polyurethanes, but a must-have feature for clear and gloss formulations.
Exterior woods experience much variation in humidity, temperature, sunlight, and wind, causing wood expansion and contraction. To stay secured it is essential for wood to expand or contract. Applying the wrong polyurethane may block your wooden object’s capacity to respond flexibly. So, check if your applying poly can ensure the required flexibility.
Drying and Curing Time
Time for drying and curing is always an essential concern while buying an exterior wood finish. If you need a project ready shortly, go for a fast-dry poly.
The longevity of coats with full resilience should be the most critical point when you judge an outdoor finish, including polyurethane. Not only the selection of a durable poly ensures wood safety, but also it saves money by reducing frequent reapplication.
What sheen you want is a critical consideration too. You have to buy different types of finishes based on sheen. If you need a clear finish reflecting the surface’s natural wood grain, you can not choose a polyurethane matte finish. You have the freedom to get a clear varathane spar urethane varnish or even a penetrating oil finish.
How Long Does Exterior Polyurethane Last?
You have to apply three to five poly coats to your wood for perfect UV protection and resilience against outdoor environments. It will last up to 12 months with the desired performance. If you want it to last more, we recommend finishing with a sealant before applying outdoor polyurethane.
Polyurethane vs Varnish
Generally, the varnish is different from regular polyurethane. It is designed for outdoor projects like decks and exterior furniture, while general polyurethane is meant for indoor use like hardwood floors or table-tops. But those forms of polyurethane that are mainly built for outdoor use (mostly made of oil) have a close relation, instead of differences, with varnish. They are indeed a form of varnish (as said above) with other polyurethane features, for instance, spar polyurethane varnish for outdoor.
A varnish is an old form of wood finish, while polyurethane is more of a new finish. Polyurethane is formulated from oil or waterborne plastic resin, but varnish is made from pure resin, solvent, and oil. The most vital difference is the ratio of solids. Varnish has a higher ratio of solids, making it less susceptible to UVs and damages than conventional polyurethane.
Polyurethane can give both – a clear finish with added natural hue or a rich amber tone. But varnish gives more tinted color only and needs more coats than polyurethane. But exterior wood polyurethanes have a decent balance between these two. Knowing what is necessary for outdoor wood surfaces, manufacturers produce a unique kind of polyurethane formulation that contains toughness and UV protection like varnish.
Will Polyurethane Make Outdoor Wood Waterproof?
Polyurethane stops moisture and water absorption while repelling water. It makes woods water-resistant but does not entirely make them waterproof. Sealing with a polyurethane or epoxy sealant, followed by exterior wood polyurethane coating, is the best way to completely waterproof your wood.
You have three ways to make your woods fully waterproof –
- Create a stunning and protective hand-rubbed oil finish (linseed or tung oil).
- Use any sealant (polyurethane, varnish, lacquer) before applying wood finish
- Complete finishing your wood with a stain-sealant combo.
What is the best outdoor clear coat for wood?
If you need a perfect outdoor clear coat reflecting the natural wood grain of your woods, you can not choose a polyurethane matte finish. You have the freedom to get a clear varathane exterior polyurethane or even a penetrating oil finish.
Can outdoor polyurethane be sprayed?
Yes, you can do that. Either you can take a spray gun (to be filled with poly from a container) or directly buy outdoor polyurethane spray. Whatever you do, you should apply thinner coats (as thin as possible) to get the perfect result.
Exterior wooden structures face relatively extreme conditions compared to indoor ones. They are prone to decay and damage quickly. Safeguarding them against rough outdoor environments has never been easy. Reducing exposure, sealing, varnishing, painting, and many DIY methods are used for their maximum protection against damage.
The use of exterior polyurethane with high-quality formulations is probably the cheapest and readily available option amongst all. All you need to pick the right one in a particular case is what we tried in our analysis to reflect.