The key is a good china bristle brush.
They are made for oil based paints. A nylon bristle brush like you use for latex paints will leave more streaks and brush lines in your job.
I use a 1 1/2" angled one. They make them with black or white bristles. I always buy the white ones. Expect to pay $12-$20 for it. If you clean it good afterwards it will last for several jobs.
For the first coat I dip straight out of the can - no thinner.
Long brush strokes.
Don't try to do it with one coat. You'll get it on too thick and have sags and runs.
Second coat I do add a small amount of thinner. Maybe a couple tablespoons thinner to half a quart.
Makes the paint flow better.
Enamel paint takes a long time to dry. So you have plenty of time to inspect your work for drips and runs. Walk around it - brush in hand and look it over for spots you missed or runs and drips and brush them out.
Second coat it the next day or at least within two days so it bonds well the first coat.
I have never painted the tin on a tractor as as I have a spray gun for that. I'm sure you would get a high quality "20/20" job (20 feet or 20 mph) that would look good and last much longer than a rattle can job.
Much cheaper too - even counting the brush.
Here's some front weights I brush painted about a month ago. Rustoleum primer and tractor enamel. Not the best photo but...
I like to brush paint.