merbau decking

Hi all, i live in northern victoria, frosty winters and long hot summers. I am going to lay merbau decking( 90x20) over treated pine joists(90x45) which are going to be fixed to sloping concrete. Approximately half of the decking will be exposed most of the year and has a northerly aspect.
Q1- Should i have a moisture barrier between the concrete and joists?
Q2- What is the most effective way to fix the joists to the concrete?
Q3- I'm stuck as to whether i should stainless screw the decking or use twisted gal nails with the dome heads, i am going to pre-drill every hole.
Q4- How far apart should you pre-drill your screw/nail holes?
Q5- What spacings should i use between the decking?
Q6- What is the best product to keep my timber looking fresh, prevent fading, staining and aging and when should i apply it, thanks Tinny.


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