When it comes to the lettering for your signature on a painting, I think it's time for a specialist brush called a rigger. This is brush with long hairs that is designed to give narrow lines while holding sufficient paint so you don't have to reload it for each letter.
It's worth spending the money on a top-quality one. You want it to retain its shape, the hairs to keep a sharp point so you're painting a line with a consistent width. For the brush have a bounce in the hairs that makes it respond to a flick of your fingers. You don't want the hairs to splay out every which way giving squiggly lines.
Get a narrow rigger rather than a bigger one. It's easier to get a fatter line by using the side of the brush (rather than only the tip) on a small brush than it is to get a fine line using only the tip of a bigger brush.
Next page: How to Hold a Rigger Brush