Treatment of wood exposed to sun and rain?

I desparately need advice on how to treat a sun deck and my wooden garage doors and patio furniture which is exposed to the elements. The front door was merely varnished when it was installed and now, 5 months later, the varnish has become brittle and is flaking. It has also faded and has ‘burnt’ to an opaque white colour. Obviously, this has to be removed thoroughly. I was told to put 4 coats of varnish thinned with turps at the ratio of 75:25%. This should dry thoroughly between coats. Only thereafter do I apply two coats of varnish. This was not done initially. I am not certain whether this really works and obviously I don’t want to spend more time and money on this task if it is not the correct treatment.

PLEASE, is there some kind person who can tell me what the real proper treatment is. I know there must be a way. After all, how do people protect their yachts and sundecks at the sea?

Thank you in advance for everybody’s input, it is going to be much appreciated.

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