Which is the most stable varnish?
Try it first: pick an option, then read the explanation.
Varnish Stability and Durability
The "stability" or durability of a varnish depends on the quality of the final film it produces. This is determined by its composition, specifically the type of resin and solvent used. A more stable varnish forms a tougher, more flexible, and more weather-resistant film. Varnishes are classified based on their solvent, which directly influences these properties.
Detailed Analysis of the Options
(a) Water varnish
Composition: Uses modern synthetic resins (like acrylic or polyurethane) with water as the solvent.
Properties: These are convenient (low odor, fast drying, easy cleanup) and form a clear, non-yellowing film. While modern polyurethane water varnishes are quite durable, traditional oil varnishes are generally considered tougher and more resistant, especially for exterior applications.
(b) Turpentine varnish
Composition: Uses turpentine as a solvent for natural resins like dammar or rosin.
Properties: Turpentine is a traditional solvent that dries relatively slowly. The resulting film is harder than a spirit varnish but generally less tough and weather-resistant than a high-quality oil varnish. It's often used for interior work and art applications.
(c) Oil varnish
Composition: This is the correct answer. Oil varnishes are made by cooking a hard resin (like copal or amber) with a drying oil (like linseed or tung oil) and thinning it with a solvent like turpentine or mineral spirits.
Properties: The cooking process combines the resin and oil, creating a highly integrated and robust vehicle. It dries slowly but cures to a very tough, flexible, and elastic film. This elasticity makes it highly resistant to moisture, temperature changes, and physical wear. Because of this superior durability, oil varnishes are the most stable and are the preferred choice for demanding applications like floors, boats, and exterior woodwork.
(d) Spirit varnish
Composition: Uses a volatile solvent like alcohol (spirit) to dissolve a resin like shellac. French polish is a prime example.
Properties: This type of varnish dries very quickly as the alcohol evaporates. However, the resulting film is hard but brittle. It is susceptible to damage from heat, water, and scratches. Its lack of flexibility and weather resistance makes it the least stable type of varnish, suitable only for delicate interior furniture.
Varnish Durability Comparison
| Varnish Type | Drying Time | Film Hardness | Flexibility & Durability | Best Use |
|---|---|---|---|---|
| Oil Varnish | Slow | Hard | Excellent (Most Stable) | Floors, Exterior Wood, Boats |
| Water Varnish | Fast | Hard | Good to Very Good | General Interior Woodwork |
| Turpentine Varnish | Medium | Medium-Hard | Good | Interior Woodwork, Art |
| Spirit Varnish | Very Fast | Very Hard but Brittle | Poor (Least Stable) | Fine Furniture (Interior) |