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.
Paint preparation is the foundation for the entire job — if done correctly your paint will perform to its fullest potential. If done incorrectly, the best paint will crack, peel, or chip easily. Make any necessary repairs before you start your painting project. Dirty walls should be cleaned, especially near the stove and sink. Greasy deposits and soap scum can interfere with paint's adhesion.
A good paintbrush is key to a professional-looking finish. "A quality brush costs $15 to $25, but you'll discover that pros aren't as talented as you thought," says Doherty. "The equipment has a lot to do with their success." Most of our pros prefer natural-bristle brushes for oil-based paints, but they recommend synthetics for all-around use. When choosing a brush, pay attention to the bristles. Synthetic brushes are made of nylon or polyester, or a combination of the two. Poly bristles are stiffer, which makes them good for exterior or textured work, but for fine interior work, Doherty uses softer nylon brushes. Look also for tapered bristles, which can help you work to an edge, and flagged tips, which help spread the finish smoothly and evenly. Brushes are available in 1- to 4-inch widths. Most painters keep an arsenal on hand to match the job. "Use common sense," says Maceyunas. "A smaller brush gives you more control, but no one wants to paint a door with a 1-inch-wide brush." Doherty recommends starting with a 2- or 2-1/2-inch sash brush. The angled brush makes it easier to cut to a line and puts more bristles on the work than a square-tipped brush.