This nifty pen from Kemper Tools holds like a brush and writes like a pen. It works on most any surface, either horizontal or vertical. This special pen is used for making consistent width lines writing/drawing with acrylics, inks and other mediums.  

Make sure the medium you are using is thin enough to go through a tiny tube with ease. Use a syringe, brush or eyedropper to fill the cup. Hold the tool like a paint brush, lightly touch the tip to the surface. The medium is released when the tip touches the surface. let the tip glide over the surface. never press down on the tip. If the medium does not release, put the enclosed wire through the cup and out the bottom of the tip. Remove any drop of the medium on the end of the wire and then pull he wire out.  Always rinse when you're finished, never leave the medium in the cup to thicken or dry.  To clean the cup and stem after each use, just soak in some warm water.  The enclosed thin metal needle can be pushed through the stem to clean out any residue.