I wouldn't over do it with the heat to the point of baking it that's for sure, solvents are good for wet varnish but won't do much on cured varnish.
There are all kinds of bio friendly paint strippers out there but as previously mentioned, it's hard to imagine that any of them won't effect the metal finish underneath. This is far from the first time someone has had to deal with a gun that got "re-freshed" by dunking it in a barrel of varnish.... hopefully someone with chime in with something that works.
I have used laminate samples from hardware as scrapers to remove unwanted substances from surfaces while minimizing the wear on the surface underneath.... might want to see if you can gently scrape off the coating? If it is a varnish it will be brittle and might crack and flake off with some gentle persuasion.