I brush painted my trailer with the Majic paint from Tractor Supply. One coat was all it took. I had done some welding on it, didn’t do any more than grinding the welds a little, slopped it on. Turned out great.
I used the TS red and rolled it on right out of the can. Very durable however the red fades after even a few months while stored inside. Easy to "freshen up" when needed
When you go to your local farm store to buy your paint, if there is an option for farm equipment or implement paint, spend the extra few bucks to buy it, its better stuff then the plain old oil base paint. If you choose a color other then black it might start to fade in 5 years, the hardener helps slow the fade. Black doesn't seem to fade nearly as bad as red, and red is about the worst for fading. Brush it on, or roll it on, that gives a thicker coating then spraying will. If you spray it, you will probably have to thin it, and that also weakens the finish. If you can get it out of the sun while you are painting it, it will smooth out better. After its painted, put it in the sun to harden better. Buy a few cheap brushes, and throw them away when the start to fill up.
Our Tractor Supply has not carried the Van Sickle paint line in at least five years. They switched over to the Majic brand. I have however seen that brand for sell on Amazon.
You do have to be careful with bright colors in the TS paint, especially the International red- the color will fade if left in the sun much. The antique tractor guys argue over this stuff at length