Hi @Trev2,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
There's no need to entirely remove the paint; you just want to remove anything that is flaking or peeling because the next layer of paint is only as good as what it has bonded to. If the paint is still well adhered, then there is no issue applying paint over the top.
Using an orbital sander, I would suggest giving everything a sand with 120-grit sandpaper. This will remove any surface contaminants as well as smooth the surface in preparation for the paint. Pay more attention to any areas with loose flaky paint. If need be, use a paint scraper to remove the flaky bits until you have a nice, stable surface, then give it another quick sand.
Once everything has been sanded, use Polyfilla Exterior Timber Filler to fill any cracks or holes, allow it to dry, then sand it smooth with the same 120-grit sandpaper
Exterior paints are considered self priming, so there is no strict requirement for them, but due to the varied nature of the surface, it would be worthwhile applying a coat of primer such as this Dulux 1Step Prep Primer, Sealer & Undercoat, allowing it to fully dry, then applying 2-3 coats of an exterior paint such as Dulux Weathershield in a colour and sheen of your choosing.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
