If you're going to paint, take Mr Spark's advice about a primer.
Better still ... don't use paint at all. Opaque, coloured woodstains are much more resistant to shrinkage and flaking. Bare wood ... then 2 coats of this.........
Indeed, you can involve water-based emulsions as an undercoat and then overcoat using a water-based gloss or satin finish. While this wouldn't be finished by an expert decorator because of it leaving a less noteworthy completion, assuming this is all you have available to you, it will be totally fine. To know more about exterior painting you can contact https://www.magicviewdecorating.co.uk/north-london/exterior-painting The Professional Painters & Decorating Service in United kingdom.