Varnishes in building works — identifying the most durable type Among common varnish types used for wood and metal finishes, which is generally considered the most durable for exterior service?

Introduction / Context:
Varnishes protect and beautify surfaces. Durability depends on the binder chemistry and film-forming mechanism. In exterior conditions—UV, moisture, and temperature swings—some varnishes outperform others.

Given Data / Assumptions:

• Comparison among water, spirit, turpentine, and oil varnishes.
• Focus on long-term durability rather than speed of drying or ease of application.

Concept / Approach:
Oil varnishes employ drying oils (often with natural or synthetic resins) that oxidatively polymerise to form tough, elastic films. Spirit varnishes (shellac in alcohol) dry fast but have limited water/alkali resistance. Water varnishes and simple turpentine-based coatings typically lack the robust crosslinking and elasticity needed for demanding exposures.

Step-by-Step Solution:

Verification / Alternative check:
Field performance and maintenance intervals for exterior woodwork (doors, rails) support oil varnish superiority in durability compared with spirit-based finishes.

Why Other Options Are Wrong:

• Water varnish: often less water/alkali resistant after weathering than oil-based systems.
• Spirit varnish: fast-drying but brittle and moisture sensitive.
• Turpentine varnish: turpentine is mainly a solvent; durability depends on binder which is typically less robust than oil-resin systems.

Common Pitfalls:
Equating short drying time with long service life; exterior work typically favours tougher, more elastic films even if drying is slower.

Final Answer: