Coach-Kote is not a varnish in the normal sense, it is a clear coach enamel. But that does not say enough about it, Coach-Kote is a long oil resin based product which means it has a long molecular structure giving it much more durability, flexibility and toughness.
With the addition of an ultraviolet filter (suntan lotion for paint), this makes the product vastly superior to conventional varnishes, especially outdoors. It also has a higher "film build" than conventional varnish so you get more protection more quickly.
Coach-Kote is a very versatile material frequently used on boats, furniture and garden furniture.
£22 for one liter
Available in store or online here: http://www.wilsonspaints.co.uk/product/coach-kote/