I am looking at painting hardiboard or mudding and then painting. Is that OK?
I don't see why not. Hardiboard has even more texture than sheetrock for mud adhesion and at least as good of moisture absorption, so mud should adhere quite well. Certainly the flex of Hardiboard is less than sheetrock, so there should be no issues with flexing causing cracking of dried mud.
But why would you want to? Hardiboard is at least three times the cost of sheetrock and harder to install.
This is from their install guide:
Finishing with Paint, Wallpaper or Texture
- Painting or Wallpapering
- If painting, apply a drywall primer suitable for high-moisture areas, as recommended by the paint manufacturer and paint HardieBacker board as you would drywall.
- If wallpapering, prime surface of HardieBacker board with a primer suitable for high-moisture areas as recommended by the wallpaper manufacturer.
- Texture can be applied to HardieBacker board in the same way as drywall.
I don't see why not if the area stays dry. I coated my stuccoed window casings (the recess from interior wall to window surface was stucco over block) 3 years ago and have not had problems yet. Originally sanded smooth it still looks perfect. wallyk said it best, the flex in the substrate, whatever that is, will rule over time. No flex, no problem.
I used dry wall mud then latex paint on cement hardibacker behind my wood stove .(stove was 14-16” away, I know that’s close). It stuck very well even with taped joints and looked good however The heat over a number of years(7) caused the mud and paint to chip and fleck off. I’m wondering if a sealer like for concrete floors, before painting, would make it stick better, in heat applications.