Heard Tom Tynan on the radio this weekend saying nothing necessary. Hardie now applying good primer at the factory, so paint away with no prep. Paint store will try to tell you to use primer b/c they're seeing primer sales way down with the increased use of Hardie products, but don't buy it.
I work for a paint company, and often times stuff that comes pre-primed (ie doors, hardy plank) come with a primer that is mid-grade at best. Often times for warranties and guarantees to be honored, the surface must be prepared right which includes a primer...but I'm not sure if they consider the pre-primed stuff.
Is it lap and gap hardie or sheet? If it is lap and gap, make sure you caulk the gaps between the planks and the nail holes as well as along seams where trim is installed.
Though they come "factoy primed", you should prime it. I would recommend getting tinted primer relatively close to the color you are planning on painting it.
If you are wanting to re-paint, just apply a solution of bleach and water to kill any mildew and then wash the house down with a pressure washer (dont get to close) or a strong water hose type sprayer. Let it dry for a day then just prep it (caulk and putty where needed) and paint.
Hardie products will get a film on them which is just the old paint. No scraping or sanding is necessary.
