Can I paint over Durock in my shower, and do I need to acid wash the durock first?

I don't want to tile because to get a look I'd like it would be too expensive.
Im going for an aged Italian stucco look. I'm three quatres finished but was wondering if I should take this extra step to insure that my primer and two coat of painted patina will hold? My paint guy says yes my plumber guy says no way no mater what I do paint will not hold up to the steam and moisture vapours of the shower.

Thanks all.

You plumber is right. Durock is not intended to be painted, it's a subsurface for glass and/or ceramic tile. You might be able to coat the Durock with modified thinset and trowel it like plaster. I don't know much about the thinsets qualities but it is waterproof.