Painting cement fibre board (Hardi Plank)

[DIY_Doctor]

Im installing some Hardi Plank cement fibre board on my garden brick shed, but unfortunately some of the planks have brown water stains from it getting wet in some of the showers. So Im considering just painting it all a similar colour. Websites recommend 100% acrylic paint for this but questions if anyone knows:

- Do I need to prime the boards first and if so with what?
- Any recommendations for exterior and suitable acrylic paints, ideally that anyones used on something like Hardi Plank/Cedral boards?

Thanks in advance