Due to the nature and structure of the Neuland FineOne marker nibs (Sketch in 0.1; 0.3; 0.5 and 0.7 mm line width,) it cannot be refilled with the RefillOne Outliner, permanent, the RefillOne Metallic and the RefillOne, Cover, white refill inks as it will clog up the nibs.
Please use the water-based RefillOne refill ink for this purpose.