Be aware that new pressure-treated wood contains mould-inhibiting additives that make it unsuitable for direct painting. Only after a year can it be painted over with paint or varnish.
Untreated wood that is to be topcoated should first be impregnated with standard base oil. Then sand with fine sandpaper before painting on with an outdoor paint.
Paint edges and details first. Take a little paint on the brush and paint thinly, otherwise the paint will run easily between the slats and boards. The paint covers better if you paint several coats.
To get a smooth finish, finish with a light swipe of the paint in the direction of what you’ve already painted. It is not visible where you lift the brush on the painted surface. But it shows where you put the brush in the wet paint.