_________________ are specially adapted for exposed works which require frequent cleaning.
🔬 Understanding the Application Requirement
The question asks for a varnish suitable for "exposed works which require frequent cleaning." This implies two critical needs:
- Durability: The varnish must be tough enough to withstand exposure to weather, UV light, and temperature changes.
- Hardness & Washability: The surface must be hard and non-porous so it can be cleaned repeatedly without being damaged.
⚖️ Detailed Analysis of Varnish Types
Let's evaluate each varnish type based on the required properties of durability and hardness.
(a) Oil Varnish
Composition: Made by cooking hard resins (like Copal or Amber) with a drying oil (like boiled linseed oil or tung oil).
Properties: Oil varnishes dry slowly through a chemical process (oxidation), forming a tough, hard, and highly durable film. This film is elastic enough to handle exposure yet hard enough to be scrubbed. This makes it the ideal choice for demanding applications like exterior doors, window sills, and furniture that needs frequent cleaning.
(b) Spirit Varnish
Composition: Made by dissolving a soft resin (like shellac or lac) in a volatile solvent like methylated spirit (alcohol).
Properties: This varnish dries very quickly as the solvent evaporates. However, the resulting film is more brittle and less resistant to weather and water compared to an oil varnish. It is best suited for indoor furniture and decorative items that do not require frequent, heavy cleaning.
(c) Water Varnish
Composition: Typically made by dissolving shellac in hot water, often with an emulsifying agent like ammonia or borax.
Properties: This is the least durable option. The resulting film is not very water-resistant and would be easily damaged by cleaning or exposure to the elements.
(d) Turpentine Varnish
Composition: Made by dissolving a resin (like mastic or dammar) in turpentine.
Properties: Turpentine is a less volatile solvent than alcohol, so this varnish dries slower than a spirit varnish. It forms a harder film than a simple spirit varnish but is still significantly less durable and weather-resistant than a properly formulated oil varnish. It is often used for high-quality interior work or as a picture varnish.
💡 Study Tips for Varnish Types
- Oil = Outdoor/Tough: Associate the slow-drying, tough nature of oil varnishes with outdoor or heavy-duty applications.
- Spirit = Speedy/Indoor: Associate the fast-drying, more delicate nature of spirit varnishes with indoor furniture and fine finishing.
- Water = Weak: Remember that water-based varnishes are generally the least durable and not suitable for exposed work.
- Think about the drying process: A slow, chemical cure (oil varnish) creates a stronger, more interlinked film than a fast, physical evaporation (spirit varnish).