Uniball Paint Marker PX-21 Fine Uniball’s PX-21 Paint Marker is permanent on most surfaces, and writes on paper, metal, glass, ceramics, plastic, wood and stone. Nib size: 1.2mm bullet tip. The Uni Paint Marker is a popular oil-based marker designed to write on any surface and retain the vivid waterproof ink colour. PX-21 has a fine acrylic bullet-shaped nib which writes a 0.8-1.2mm line. Permanent on most surfaces and writes on paper, metal, glass, plastic, wood and stone. It also contains Heat resistant, water proof and fade proof ink.