After much deliberation I have decided to use DuPont Imron Elite for the topcoat. Rob Loneragan from Searey Australia has also used this product with good results. It has been used by a number of aircraft painters in Australia. It is a 3:1 mix plus 15% of flex agent added. It will be sprayed directly over the Poly Spray without a primer coat.
Time will tell if it was a good choice.
Imron Elite topcoat with activator and flex agent.