There are a number of ways to do this, including
But the most advocated method is using a wash. Below I describe a variation of the wash method that I use.
|First decide on a panel line color. For a very anime look, straight black is sufficient, but if you want a more realistic finish, cut back on the contrast and use a grey or dark grey. Even better, a darker version of the base color. Here the Valkyrie was panel lined with neutral grey.|
|I love using artist oil paints for doing panel lines. For demonstration purpose here I use Ivory Black, thinned with Odorless mineral spirits (OMS). Thin it so that you hacve a little pool of "dirty thinner". In the picture, I used a brush to drawed in the panel lines. At this point there's no need to be careful, as long as there is paint inside the groove it's ok.
Another method, if you have a *glossy* base coat, is to utilize capillary action. Dip the brush in your wash and gentally touch the panel line. The paint will flow along the groove for a distance. Repeat where it stops. Without a glossy base coat this doesn't work very well because the paint won't flow very far.
|After the thinner evaporated in about 1/2 to 1 hour, I take a napkin and wipe the excess paints away. Make sure your napkin's wiping surface is flat, so that the napkin does not go into the panel groove and remove the panel line paints inside it.
If you experience difficulties in getting rid of the paint, a *little* OMS on the napkin will do the trick.
One thing to note is the glossiness of the base coat. If you want a clean mech, the base coat should be glossy or semi-glossy, because a flat surface traps paint and you'll have a hard time removing the excess. However if you're planning to do a dirty, weathered mech, this is ideal. You'll get some soot-like effects around the panel lines.
|The piece after wiping. I'm going to weather this kit, so there's no need to get the piece ultra clean; here I have clearly defined panel lines with some dirt around them. If you messed up a little and wiped away some paints inside the panel groove, use a #000 or #0000 brush to do a little touch up.
Most of the rest of the world uses enamel paints instead of oil, using slight variations of the procedure above. With enamel replace OMS with straight mineral spirit. The reason I use oil is because with enamel paints, you have to use thinner to wipe away the excess, and the probability of lifting paints from the panel groove is a little higher. Moreover I like the soot-like effects oil produces. But it's all a matter of personal preferences, there are more than one way to model!