Which finish is most resistant to moisture and heat?

Polyurethane varnish is particularly known for its robust properties that make it the most resistant to both moisture and heat among the options listed. This resilience stems from its chemical composition, which creates a hard, protective layer over the wood that effectively seals it from environmental factors.

When applied, polyurethane forms a durable film that is not only waterproof but also withstands higher temperatures without degrading. This characteristic makes it ideal for aviation applications where wood components may be subjected to varying weather conditions and potential heat exposure. Additionally, polyurethane is highly resistant to chemicals, which further enhances its suitability for protecting wood in demanding environments.

In contrast, while oil-based paint can provide a certain level of moisture resistance, it does not offer the same durability and heat resistance as polyurethane. Shellac is typically less durable than polyurethane and can be more susceptible to heat damage, while water-based finishes, though easier to apply, often do not provide the same level of moisture and heat resistance as polyurethane varnish.