Martin - I've never tried to add chipping effects with a paint brush. Perhaps it's time I learned how to do that. Any suggestions on placement of said paint spots?
First off look at real locos and see where the chips happen.
Likely are places with constant or normal contact.
Scratches can also be done with he Vallejo.
I was also thinking there should be some shiny wear spots where the operator would grab on and step.
These can be done with a graphite pencil to give the dark steel color.
I have recently discovered the MIG weathering pigment dark steel and really like it.
At least by using acrylic if you dont like it you can remove it!