Well, it all depends. A basic rule of thumb is never hard over soft. Conversion varnish is harder than nitrocellulose lacquer (if that is what you have). If it was to work (and I'm not saying it won't) prep will be all important. Following all the normal cleaning and prep, I would seal with a vinyl sealer.

This is not a job for someone inexperienced using conversion varnish and professional spray equipment.. You would be well served by having a finishing professional (not a painter) do it. Conversion varnish must be sprayed with equipment similar to what auto body shops use, not junk-in-the-box store spray guns.