the lines are very sharp. the approach that i used was 'masking' and i have kind of perfected it over several boats.

step one, mask around the white wood/feature line (not stained) and apply a coat of epoxy only over the feature line.

step two, when the epoxy over the feature line is cured, mask the feature line itself (masking on top of the epoxy) and stain the surrounding wood. when staining around the masked feature lines, make sure the stain rag is sort of dry and not dripping wet.. after staining, keep making tape in place and apply a seal coat of epoxy to lock the stain in place (this is critical or else the stain will move). then remove masking tape (for thin feature lines, this step may require two passes, one piece of masking on one side of the feature line, another masking on the other side of the feature line.)

step 3, inspect work and clean up any stain that may have gotten onto the feature line. this clean up is pretty easy because if it has bled over, its on-top of the epoxy, not underneath.

step 4, glass

you can try it out on a sample piece to get a feel for what i am describing above. but it results in very sharp lines in my opinion.