As a person who plays and paints Eldar for Warhammer 40k I understand how much of a pain it can be to paint panel lines. It’s one of those things that is super easy to mess up with one small slip of the paint brush. And fixing it is just a royal pain in the butt.
The following method was something I figured out a few years back that makes it much easier to do the panel lines or hard lines on models because it’s super easy to fix of you do happen to slip up.
Supplies:

As you can see it does not take a lot to do.
1. The model to be lined
2. Dry pigment. I prefer Gamblin, I suppose you could also use water color paint.
3. Small container, I personally mix up larger batches since I have a lot of panels to line. You could also use a pallet with a cup.
4. Two brushes (not pictured) one for lining and the other for cleaning up. I used my Winsor and Newton Series 7 size 00 and 2 for this.
I highly suggest a light clear coat or at least letting your model either set for several hours or blow drying it to ensure the paint is fully cured. This will help from having paint coming off during the clean up.

Step 1: In the cup or pallet add a little bit of water. What you see in the cup was a bit more then I really needed, so I ended up making a larger batch then I needed.


Step 2: Scoop up a good amount of pigment, here I just used a flat edged blade.

Step 3: Dump it into the water

Step 4: “Wiggle, Wiggle, Wiggle Yeah!” Shake it up you will be doing this often while lining. The pigment trends to sink.

Step 5: Make sure your paint area is ready. Remember what I said about making sure it’s fully dried/cured or optionally clear coated. I’ll show you why soon because I sure didn’t….


Step 6: Apply the dry pigment mix into the lines, you can be neat or not because you will go through and neaten it up.

Take the larger brush, get it really wet, and go over the areas that you just finished lining to neaten up the area. Now if you notice the dark spot on the paint that was from my freshly sprayed basecoat coming up. Remember what I said about making sure it’s dry, cured or laying a clear coat? This is why.
Now that you have laid down the dry pigment give it some time to dry completely. Now you can spray a light mid-coat clear or final clear depending on where you are in the painting process. If you don’t and the model gets wet the pigment will become loose again.
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