Tombow Mono 01 Waterproof Pigment Ink Pen (0.1mm tip size) for Watercolor Painting
Tombow Mono is my go-to disposable fineliner for drawings that will be painted over later. It is waterproof and lightfast, it dries very quickly so I can move on to watercolor painting after a few minutes. Sold individually. This pen produces a fine line, labeled as 01 size. It is capable of thin details for those who work small. I use this pen for subjects drawn on artist trading cards or 5x7" paintings.
Like all disposable fineliner pens (such as Uni Pin, W&Newton, Micron's non PN version etc.) it has a very fine felt tip in a tiny metal needle tube, which can be worn down by pressure when drawing. When using fineliners on textural cold press watercolor papers I recommend a slow and light touch to help avoid needle drag on the dips and valleys of paper, and to help your pen be able to complete a few drawings with the most precision. For non-disposable options I recommend a technical pen like Rotring Isograph and R&K's Sketch Ink bottles, but those require more maintenance, can leak and have to be cleaned out between 1+ week storage so I only suggest them for highly active artists blowing through a lot of disposables. I talk more about pens, brushes and other watercolor related supplies on this page.
While this pen does come in larger tip sizes, I almost exclusively use the 01 size in my paintings and just go over edges again where I want a thicker line width. You can see this pen in action in the video below around 5:01 where I use it to go over my pencil drawing before soaking the linework with water and paint.