I'm a HUGE fan of the Case IH Iron Guard paint. I've used both quarts and spray cans, with little difference. Here are two I painted myself with a top-feed spray gun from Harbor Freight, and Iron Guard in a quart, no hardener, 1965 Lo Boy, and 1966 IH Cub:
The tractor below I just did last summer. I had a friend, retired auto-body guy, who wanted to do the sheet metal for me, so the hood, lights, fenders, batt box, were all painted by him using Iron Guard with hardner. The rest of othe tractor was painted by me with Iron Guard spray cans.
You can see a little difference in the paint with hardner, mostly just in the gloss. However, since they are sheet metal pieces against the cast iron pieces, it looks more like differences in texture than in the paint itself.
Regardless, YES, you can get great results using paint with or without hardner.
As for the Valspar paint, I used it one time to paint a mower deck, and swore that I'd never use it again. It does not cover well, and takes FOREVER to dry. Even the dry finish is soft. My recommendation would be to stay away from it. After all the work of cleaning, degreasing, cleaning, degreasing again, dissassembling, reassembling, sanding, grinding, etc., I'd use the best paint I could afford because the paint is the easy part!