What is the difference between spar urethane and spar varnish?
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Spar urethenes and spar varnish are different formulations. Urethene, naturally is a urethene based product, whereas varnish in the traditional formulas had linseed oil as well as various hardeners.
On the old formulation the varnish never completely dried. This was so it would not dry hard and crack when exposed to great heat, UV light and salt water. (Try putting a hand on a wood rail or spar that has been exposed to several houirs of direct sun.) So it is a bit soft. If you use spar varnish on lawn furniture for instance, and sit on a sun warmed bench for a while, you will have to peel yourself off, and will see a print of the fabric pattern on the bench.
I think the urethene product performe better in this last example, but it is also formulated to be very stretchy and flexible as well as resisting UV light which destroys most paint and varnish finishes. It sacrifices hardness in this same way as the spar varnish. Both types need to be sanded and re coated fairly often to retain their protective properties. Linseed oil (a component of spar varnish) turns dark in exposure to UV - which is why many antiques are much darker now than when they were first sold.
Spar varnish is an older formulation of varnish vs. Urethane Varnish is a modern formulation of a tough clear coating used primarily on wood surfaces exposed to outdoor conditions. Marine varnish is just another name for a similar product -- naturally intended for protecting wood surfaces under marine (wet)conditions.
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