I've used liquid letters to touch up the very worn/faded Vipermax logo on my rib.
It works fine. I did one side in Spring 2012 and it hasn't worn or deteriorated at all so far.
Only thing I'd say is that you've got to be quite careful to get a nice smooth finish. Unlike normal paint, which smooths down after it's brushed on, the Liquid Letters just seems to dry with the brush marks still in it. So worth getting a smooth finish as you put it on and using a suitable brush. (I used a few different types of cheap pencil brushes). If you want a before and after picture, I can find one.