I was working with a user recently who had used hairline outlines on a number of lines in a CorelDRAW project. When output, the results were not as desired.
The idea behind hairline outlines is that they will output at the minimum possible width on any given output device. On a 600 dpi printer, they would be a single pixel wide and thus 1/600 of an inch. In short, barely visible. Yet your screen is lower resolution so that same outline would look thicker on your screen. What you see is definitely not what you’ll get. Look at the screenshot shown at right. Hairline looks thicker than both the .5 points and 1 point outlines!
If you want the outlines to output at a specific width, choose that width! Corel even changed the default from hairline to .5 points (or an equivalent in your favorite measurement system) in recent versions of CorelDRAW. Try to avoid hairline outlines unless that is exactly what is required for a specific project or type of output. For example, output with a “cutline” might require a hairline width.