A couple comments...
Yes you can get a fast-drying polyurethane clear in a spray can that gives a pretty good durable finish. BUT, the color coat should be fairly thin and COMPLETELY hard or you could have the problem that you described. Lightly sand your color coat (at least 48 hours after spraying) and let it cure for a couple weeks. Then spray 2-3 coats of clear poly waiting no longer than 1-2 hours between coats (the previous coat of clear should still be slightly tacky). Let that cure for a couple weeks before final sanding/polishing.
Lacquer is a much easier way to go but ALL of your paint (color coats and clear coats) should be lacquer to get the best result. Lacquer "melts" into the previous coats giving you essentially one solid coat. Don't try to spray lacquer over enamel (unless you really know what you're doing). You can spray coats as little as a couple hours apart, but the longer the better.. Again, allow a couple weeks to cure before final sanding/polishing. On my custom guitar builds, I allow at least a month before final sanding/polishing.