I don't fiddle with lines in the middle of surfaces (I try not to put them on my designs).
For edges, I just try to tone them down. If I have a lighter, close color match I'll use that on an edge (simple water-based markers), for black parts I'll use a black Sharpie, for everything else just a soft pencil in 25, 50, or 75% gray shade (craft store item).
Yogi
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