Contractors apply several layers of paint to achieve a suitable finish. They leave the first coat to dry for the recommended time and apply one or more finishing coats. They might choose paints with special qualities for different types of room. Manufacturers have developed paints for bathrooms or kitchens that have good resistance to moisture for example. When they have completed painting, they clear away any equipment and restore the area to its original condition.
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.