I've seen this done many years ago by a guy who did all sorts of jobs where I lived. My dad caught him painting his boss's roof and he was watering down the paint. We've had a hard time trying to get a painter for our house. We've had a guy that had been remmended by a neighbor and he hasn't showed up to paint for 2 weeks. I want to call him and my husband says no. I'm 69 years old and I'm about ready to go and paint our 2 porches, I've painted all my life and all the inside of our home. Maybe that's the way it should be with everyone that can paint pretty good.
Oh, where to begin? Let me start with 'watered down paint'. 25-50% before the material gets to the site? Impossible. You would basically be painting with water at that point. It would be less of a hassle, and cost, to simply use proper material. You would be forced to apply three coats instead of two, as the coverage would be horrible. Whatever cost you think might be saved in materials would be lost in labor.