Best hardwood to use on a painted conservatory .

[Giff]

I am not an expert but have used Iroko for an outdoor project and primed it with Crown alluminium primer then F&B oil based......paint seems to have stayed on but need good dust extraction / masks when machining.. Giff

 

[Lowlife]

The only thing I'd say about Douglas-fir is that it can be very variable quality, I've used lots for boat building and will now only use it if I can select it myself. Good quality slow grown D-f is nice dense stuff, 20+ rings per inch is strong, stable, and very rot resistant, but some I've had has obviously been grown very fast, it has far fewer rings per inch and has been very light and weak.

Western Red Cedar might be a good choice for cladding, excellent rot resistance for a softwood even if you don't treat it, but it also takes paint and oil finishes well, and it's not expensive. WRC on a hardwood frame would probably work well.