The most durable varnish is Oil varnish.

Oil varnish is a type of varnish that is made by mixing oil (such as linseed oil or tung oil) with a resin (such as polyurethane or alkyd resin). This mixture is then applied to a surface, such as wood or metal, and left to dry and harden. Oil varnish is known for its durability and long-lasting finish, making it the ideal choice for protecting and enhancing various surfaces.

Here are the reasons why oil varnish is considered the most durable:

1. Durability: Oil varnish creates a strong and durable protective layer on the surface it is applied to. It forms a hard film that is resistant to scratches, abrasion, and other forms of wear and tear. This durability ensures that the underlying surface is well-protected and can withstand daily use and exposure to various elements.

2. Water resistance: Oil varnish is highly resistant to water and moisture. It forms a barrier that prevents water from penetrating the surface and causing damage, such as warping, swelling, or rotting. This makes oil varnish an excellent choice for surfaces that are exposed to moisture, such as outdoor furniture or wooden decks.

3. UV resistance: Oil varnish also offers excellent protection against the damaging effects of ultraviolet (UV) rays from the sun. It contains additives that act as UV absorbers or blockers, preventing the sun's rays from causing fading, discoloration, or degradation of the surface. This makes oil varnish suitable for exterior surfaces that are exposed to sunlight, such as wooden doors or window frames.

4. Flexibility: Oil varnish has a certain degree of flexibility, which allows it to expand and contract with the underlying surface. This flexibility helps to prevent cracking and peeling, especially in areas where temperature and humidity fluctuations are common. It ensures that the varnish remains intact and provides long-lasting protection.

5. Ease of maintenance: Oil varnish is relatively easy to maintain. It can be cleaned with mild soap and water, and minor scratches or imperfections can be repaired by applying an additional coat of varnish. This makes it a practical choice for surfaces that require regular cleaning or touch-ups.

In conclusion, oil varnish is the most durable varnish option due to its strength, water resistance, UV resistance, flexibility, and ease of maintenance. It provides long-lasting protection and enhances the appearance of various surfaces, making it a popular choice for both interior and exterior applications.