I have a 2-year-old guesthouse, covered in stucco, which has never
been painted (Southern AZ). I'm going to be painting it this fall, and
I'm seeking advice as to the extent to which it needs to be power-washed
prior to
cleaning.

Since its just a guest house (smaller), I'd P/W it just to make sure I had a
nice clean base for the paint.
When we lived in TUS, we got our paint from KM Universal at 29th and Palo
Verde? or maybe Alvernon.
I think their name now may be Kelly-Moore.

The specs on their stucco paint contained more of the acrylic than the stuff
from the other dealers like Sherwin Williams, Dunn Edwards and HD.
Five years later, it showed no fading, quite impressive in the S/AZ sun.
We also used a product from them called 'stucco seal' as a primer coat.

R