The Gundam Marker GM301 by GSI Creos contains a very fluid, oil-based, gray ink, specifically designed for panel lining of gunpla models using the principle of capillary action! Panel lining is the procedure by which the grooves of gunpla armors are inked, putting them in evidence and giving them a feeling of higher thickness and a more realistic look. Putting the tip of the marker on a point of the groove, the ink flows out along the groove itself (by capillarity) or otherwise automatically fills a large part of it! If a groove is not completely filled, just repeat the process on other points. The excess ink can then be removed with an eraser or rubbing the area with a cotton swab. Erasures can be made also using Gundam Marker GM300 (sold separately). It's a fast and fun method to perform panel lining! The ink is particularly suitable for gunpla plastic, but you should not use it directly on other paints, because it would dissolve them. Black color is ideal to panel line gray, blue or other dark colored components (anyhow, black goes with everything).