Painting - painting stucco exterior

I have 25 year old house that I purchased last year. 3 walls are stucco and they don't look like they have ever been painted. There are large stains from a storage shed that was against one wall and various stains in other areas. I will be renting a power washer and patching a few hairline cracks. I purchased Sears Weatherbeater exterior flat paint and, on the advice of the paint dept, I purchase Zinsser Cover-Stain oil based primer sealer for the stucco.

I've been told by a co-worker that the stucco needs to be wet when I paint so that the stucco doesn't "suck up" all the paint. Everything I have read says surfaces must be dry. Was he pulling my leg or is this correct? This is the first time I've ever painted the outside of a house and I want to get it right.