I tried rubber cement and it left a ragged line. I was told that the cement can also attack plastic so I guess I was plain lucky!
The Humdrol stuff is good for sealing the lines on masking tape etc. I use it on the locater pins, holes in kits where parts attach so I don't have to scrape the paint away.
It removes easier than rubber cement as well. Generally I apply it with a cotton swab on large areas or a toothpick depending on the application, but it can be removed from brushes with a thinner I believe, just not sure what though!