I don't know how easy it is to use but I've seen some really great touch up jobs with a syringe with a tiny brush at the end
Touch-Up Paint Syringes, 2 Count - Griot's Garage
best thing to do is get one of those and get the touch paint pen as well ... I just can't find myself to trust the pen because it rarely leaves a smooth finish ..
if not time to call The Junkman