In the Global Settings section under Printers double click on your printer in the list. In the properties, windows click on Advanced in the top right. In the Advanced window look for the Bleed option at the bottom left. The Windows default bleed for printers is 30 pixels. This means that Windows will enlarge and prints sent to a printer by 30 pixels to make a borderless print. Booth lets you adjust the bleed and defaults to 0. You can adjust this up until the white line goes away. Normally about 15 to 20 works well but if you have to go over 30 you may need to check to see if the paper is loaded correctly.