G Project - Panel lining! An easy yet effective way to give your models that extra bit of flair. Here are some of the more common options: 1) Panel-lining Gundam Marker (スミいれ用)-
Gundam Panel Lining Tutorial | Rants Of A Gundam Addict!
The Gunpla Panel Lining: Gunpla's Guide to Markers and Paints | One Map by FROM JAPAN
Mr. Hobby Gundam Marker GM01 Black (Fine Tip For Panel Lines) – A-Z Toy Hobby
Gundam Marker Extra Thin Type for Panel Lines Set (Renewal) | HLJ.com
Gundam Planet - Pour Type Gundam Marker for Panel Lines
How To Panel Line a Gundam Model Using a Gundam Marker Pen – Gundam Express Australia
Panel Lining for Gunpla 101 - Gunpla 101
Panel lining options Tamiya vs Gundam Markers - YouTube