Panel lining for BJDs?

If you don't trust yourself painting them on at all, I'd argue there are three options (hard to easy):
- carve the lines in first
That allows you to have a groove where the paint can just flow in. However, that means you need to be calm enough with your hand to finely carve in those lines and it also means permanently modding the doll.

- use masking tape to emulate the lines
There is some very fine, very flexible masking tape out there that even works on curved surfaces and comes in different colors. You could technically stick that on to make the panel lines.
It is easy to remove, but it would look raised on the surface vs a drawn line. The very thin tape can also be a bitch to handle, and it can easily be torn off of you keep tugging at it.

- use a watercolor pencil and a flexible ruler to draw on the lines
If you want them to pop even more, you can then still gently (after sealing) use some watered down acrylics and a brush. The pencil is easier to handle and easier to erase, until you have a line you like. Sealing it allows you to enhance it without destroying your work.

 

In addition to what Ara said, you could also use masking tape to mask off the areas that are *not* panel lines, paint/blush/however you prefer to add the colour in the line area and remove the tape. When using paint, you might want to let it dry first, though. This should give you nice and uniform lines without any shakiness.

 

My understanding from your post was a bit different from the others, that the doll is a robot sculpt with already inset panel lines, and you want to highlight them? Feel free to disregard if I'm wrong.

My suggestion is to use a diluted paint and do it how custom dice are painted: paint a section of the groove, wipe away anything that escaped, repeat.

If you'd prefer a pen or marker, you can get acrylic paint pens/markers at just about any art or hobby store that should work just fine

 

@dm_sof Whoops, you might be correct. For some reason I just straight went to "oh they are customizing a doll to be a robot", because IF there are panel lines already carved on the sculpt there is no reason to go any other route than doing the wash.
So yes, if that is the case I second this. You might worry about your hand not being steady enough for that but honestly, it's not needed. You just need to get it into the groove and then go wipe wipe at the excess outside the grooves for a clean look

 

Have you considered Mr. Hobby's Panel line wash/paint?
They're acrylic based and can be used like acrylic paint.
Panel Line Accent Color - Black 87131


There are vids that compare different panel line washes, too.
I hope this helps!