Actually, the old nitrocellulose lacquers were easier to use and easier to touch up, but they didn't resist chemicals very well. My dad was the foreman of a finishing shop for 35 years. I worked with him since I was old enough to sandpaper...
He's gone now 20 years, no surprise he died of lung cancer, but he always said he retired just in time because the old material was easy to use and it cured by solvents gassing off primarily which is a simple process. Those old lacquer products were being replaced by catylized material that was much more chemical resistant than the old stuff, but harder to get a perfect result with. I'm sure the newest chemistry is improved from it's earliest versions. Early conversion varnish was difficult to do well, but hard as hell when it dried.
The 2K was compatible with rustoleum. That means you can lay down a simple solid color cheaply, and use the 2K to get improved chemical resistance in your top coat. Of course a professional job is always better for many reasons...
You have to be careful topcoating any paint that gets effected by a solvent. Sometimes the solvent (like E10 fuel) softens the underlying paint and will react when the top coat hits it. I've heard it called shrink, reaction, aligator, wrinkle, etc... My roadster tank has the exact problem right now. It's gotten hit with brake fluid and fuel and the last time I painted it, the new paint reacted, sending me back to square 1, so I need to strip or sand down to bare metal again to get past the chemically effected paint layers... It sucks, but that's how it goes.