I have tried almost everything ever mentioned on this forum regarding clearcoats. None of them seemed to work well for me. Always wearing off quickly or not giving a good result from the start.
I have recently been using an automotive clearcoat paint. It is a catalyzed urethane and so far has been holding up very well.
After polishing the cases, I spray three coats with a small airbrush and let it cure. Very hard finish once cured.
This paint is not cheap, so unless you just happen to have some available or you need to make a LOT of pens, it is not a cost effective option to start from scratch.
MAJOR ISSUE with this paint is the fumes and over spray, ie. iso-cyanates. I don't have any references for MSDS sheets but this stuff should only be applied while wearing full coverage clothing and respirator or preferably, a supplied air breathing system.
If you know any car painters, maybe you could work something out when he's doing another project.
Just a bit of info, FWIW.