There are two distinctly different types of "Technical Pens". One consists of two spring steel nibs. The space between them is adjusted by a screw, and the distance between them is what controls the width of the line. Spacing varies with the pen, but probably 0.01" to about 0.125" would be average.
The second type are similar in appearance to normal pens. They usually consist of a cylindrical ink reservoir in the handle and a tip assembly. The tip assembly is tubular and the diameter of the tube determines the width of the line.
The first type work with just about any kind of liquid. The second are designed for ink but should work fine with thinned paints. I know several people who use both types for detailing and panel lines.
I always used Koh-I-Noor Rapidograph pens when I was drawing. They work well but are very finicky. They absolutely, positively MUST be kept spotlessly clean or they will gum up and fail. There is a wire that goes down the tube that acts as a cleaner and any ink (or paint) that is left to dry will cause it to stick in the tip and it will usually break off when you try and pull it out. The caps are pretty airtight though, so leaving it for a day or so should not be a problem.