Where two new paint colors come together on a single surface, it's practically impossible to keep a straight line between them while painting freehand with either a brush or a roller. To get a straight line, use a carpenters' level and a pencil to draw a faint line on the wall. Then, align masking tape with the line across the wall. Peel the tape off the roll a little at a time, and press it to the wall with your thumb. Don't pull the tape too tightly as you go, or it may stretch and retract once it's in place. To keep the paint from seeping under the masking tape, use the bowl of a spoon to press the tape tightly to the surface.
Don't leave the tape on until the paint is dry. If you do, it may pull the paint away from the surface. With latex paint, you only need to wait a half hour or so before peeling off the tape. With alkyds, two or three hours is enough. The paint can's label will tell you how long it takes for the paint to set completely.
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Masking tape is useful for protecting trim around doors, windows, built-ins, baseboards, or bookshelves. When you're brushing or rolling new paint on the wall, you won't have to slow down or worry about sideswiping the trim.