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I use a draftsmans ruling pen to do the ink lines; I've used the Rapidograph type pens, but they are generally more work to keep functioning than the two blade ruling pen; my favorite being the jack knife style with an articulated joint for easy cleaning. I can alter line widths and colors very rapidly with the two bladed ruling pen. The pen should be filled with a brush or eye dropper, checked for drips, and tested. No dipping, it's rude.
The straight edge needs to have a "stepped" back, to keep the edge above the line and away from the ink. Cork, as seen on commercial rulers, or even several layers of masking tape works. This is still free hand work, aided, but practice is needed.
Aesthetics. This is the tough part, but I've seen too many mats that overpower the work they are supposed to enhance. This is the true "ART" of framing; doing your best work in such a way as to not be noticed, while enhancing the art object. Washes must be subtle, washes, you know. The lining needs be subtle, muted and diluted, inks and colors.
Bron