A lot of good advice here so far.
1. Valspar from tractor supply completely unacceptable by automotive standards. However, with proper surface prep and primer underneath, it is probably better than what the tractor had from the factory (see rusty Kubota thread).
2. If rust protection is your number one concern use a good epoxy primer, then paint. If used properly, it will also provide superior paint adhesion.
3. There is no product that will substitute for proper surface preparation. Bare metal should have 80 grit sanding scratches. A wire brushed surface is too smooth for proper adhesion. Always use a good wax and grease remover before paint (never lacquer thinner, mineral spirits, acetone, or anything else).
4. Orange peel is usually caused by improper gun setup or spraying technique. In your case, the ratio of thinner to paint is also a likely cause. Valspar does not give any recommendations for the paint/thinner ratio, so like most people, you are probably using trial and error. I would have to check my notes, but I think I ended up using a 6:1 ratio, last time I sprayed tractor paint. I don't recommend Valspar, but go price a gallon of red single stage urethane and Valspar from TSC might not sound so bad after all... If the tractor is worth it, use good paint.
5. Do you need a good gun? The Divilbiss is a decent middle of the road gun. There is no question that you would be able to produce excellent results with it. Most of the pros I know are using IWATA or SATA guns for color and clear. If you end up getting more into automotive work, you could keep the Divilibss for primer and then get an IWATA for base and clear.
