You need to talk to your paint supplier about the system you are using. In general the very best paints available today are two part, catalyzed urethane color coat/clear coat SYSTEMS. This stuff is literally like coating your car with hard urethane plastic that will hold it's finish and is color fast indefinitely. Anything else is second best.
If you decide to use lacquer, you most definitely will need to clear coat it. If plan to color sand and polish the finish, you need a coat of clear thick enough so you never touch the color. If you do sand thru to the metalic, the job will be ruined and you will need to repaint.
you have to use a laquer clearcoat over laquer,if you use one of the enamel types it will never dry.I had a so-called expert work for me one time who laquered a saab then cleared it with urethane clear, three weeks later it was still like a wet candy apple,couldn't even touch it.I fired the expert for a few other expert jobs,then had to find him to see what he had done to the saab.The clear would dry nicely in the hot sun but by morning it was wet again,I had to do it over myself,now I don't paint saabs anymore
In the days past before base coat/clear coat systems were available, I use to do a lot of laquer jobs with dau75 clear over them. I never had a problem. You just have to remember to add the right amount of hardner. If you use to little, it will stay soft and if you use to much, it will get gummy.
Tom; your experience sounds really strange. i have used the urethane stuff over every conceivable base (lacquer, Testor's model enamel, bare styrene models, urethane, every kind of primer imaginable, house paint, chrome, polished aluminum, etc., etc. Would even paint my dog if he didn't shed and waste the paint!). I am really sloppy about mixing the hardner and have NEVER had any problems like you describe. Must have been the Saab. Buy real American steel next time (preferably pre-'70) and you won't have any trouble!