I can not see why anyone would want to scrape/sand the surface of
Chrysotile asbestos fibre soffit cement board - it is perfectly safe
stuff in terms of wet sheet removal, but not so safe when treated in
such an abrasive manner :-)

Soffit boards are often black with mould around house vents, I suspect
humid air exiting the house and condensing on their surface.

Conventional undercoat & gloss tend to act like a plastic bag if one
side is porous to moisture, they will bubble & flake.

You basically want a white stain which is highly permeable. Prehaps
Sikkens onol undercoat on its own would do, it is white, MVP,
overcoatable. I would not use the top-coats because whilst they are
overcoatable every 5-12yrs (depending on which one) after about 4
overcoats you need to restore to original surface. They do "wear" by
chalking rather than flaking. That is probably so far into the future
it does not matter, but you want a paint that can breathe because
soffit boards do suffer moisture loading from the other side.