Straight lines and bumpy surfaces usually don’t mix well. No matter what type of straight edge you use, drawing along an uneven, textured surface usually produces uneven results. Leave the drawing out of the process by using a chalk line. In preparation for the chalk line, mark 3 feet from the ground on either end of the wall. Tack or tape one end of the chalk like to one mark point, and then pull the line, keeping it taut, over to the other mark. Secure the line onto the other point before plucking the chalk string against the wall in the center of the line. This is called “snapping” the chalk line, and creates a perfectly straight line.
If the wall has drastically bumpy, cracked, cratered or generally wildly uneven areas, you’ll need to fix the drywall before your straight line efforts will succeed. Before working with your wall, clean the wall with soap and water, and allow it to dry. Fill in cracks and craters with joint compound, and wipe away excess. Use a drywall or pole sander to smooth away wall texture. After you’ve smoothed your walls, vacuum away all the resulting dust with a hand vac. Mix premixed joint compound with water until you get a creamy consistency, and then apply the watery compound with a broad knife.
Often, a wall isn’t a perfect rectangle or square. Instead, each corner is off 90 degrees by a couple of degrees or more. In this case, measuring up by the same amount on one side of the wall can produce a line as uneven as the wall. If you have only a slight slant, you can use a laser level to guide your line. Hold the level against the wall where you want to make the line and have a friend draw along the line, or use mounting hardware to leave it against the wall while you create the line yourself.
Setting an even line along an extremely slanted wall can mean trouble for your project. If you just use a level, you could end up with a line that’s technically perfectly even, but is obviously not parallel to your floor or ceiling moldings. Before you start setting any lines on your wall, figure out if it’s too slanted for a level line. Start at 3-foot measurement from the ground at one end of the wall, and tape a line of dark string to the measurement. Pull the line over to the other end of the wall, and then repeat the measuring and taping process. After you’ve set up your string, set up your laser level and compare the two lines. If they are drastically different, use your string line instead of the laser line.