All good suggestions, but before you decide check out the value and durability of Spar Varnish... as the name implies, such varnishes are made for hard, everyday life. One site states:
"When a varnish is made, the ratio of oil to resin can have a dramatic effect on the way the varnish will behave. For instance, using a small amount of oil and a large amount of resin will produce a very hard but somewhat brittle finish. Obviously, this is not suitable for outdoor applications since we need an outdoor finish to be flexible. So what makes more sense is to create what is known as a “long-oil varnish”, that is, a formulation that contains a greater percentage of oil. The extra oil results in a softer, more flexible finish that will not crack when the wood expands and contracts..."
The tupe of oil in a really good spar varnish will always be tung oil (rather than linseed) and the resins used should be phenolic modified alkyd resins and the contents list on the can should also reference UV protectives. There are a number of good spar varnishes available, but don't skimp on cost. As usual... three or four evenly applied thin coats with a little fine greit sanding between each is far better than one or two thickly applied coats...