Painting shields help you work faster and more neatly, but you don't need to buy a specialized tool to paint like a pro. A wide-blade drywall taping knife works just as well and is much more durable than the average shield.
A 12-inch-wide knife makes an ideal shield when you're painting alongside molding, near ceilings, and around doors and windows. When painting smaller objects, use a 6- or 8-inch knife. For best results, wipe the knife blade clean after each brush stroke.