[KQ400Pilot]

I would remove each board, then prime and paint each board individually.

Do this even if the boards have been pressure treated.

Shake sand from your hand onto the wet paint, then spray a thin coat over it to seal the sand onto the previous coat. The sand will give you some traction.

Painting pressure treated wood will encapsulate the chemicals in the wood, making the wood both last longer and providing a barrier from them and you, the environment, kids, etc. The chemicals used for pressure treating the wood, although recently changed and less toxic, are still chemicals that you do not want to touch. Wear gloves while handling the wood.