[Lloydy]

Tamiya model 'semi gloss black' acrylic paint is excellent - product code X-18, and the thinner is X-20A. Although there is a thinner the paint is also water soluble, and comes in tiny pots available at most hobby / model outlets.
I don't paint over normal wear and brassing, but I have tidied up some old cameras that have scratches and loss of paint on the film door hinge. I apply the paint sparingly with a fine brush, wait for it to become touch dry, then lick a finger and gently rub the new paint. This flattens any brush marks and blends the edges to the original paint, being water soluble the paint surface comes off on your finger. It also allows you to adjust the satin finish a bit. I repaired a very battered Chinon film door and the repair is just about invisible.
The paint is very durable once fully hardened.