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.