If it is a permanent structure e.g. coop I would really suggest using a rot-resistant lumber like cedar, or pressure-treated, for the portions actually in contact with the ground. Anything else will not do much good, the wood will rot out in short order no matter what you paint on.
For a tractor, where you can turn the thing over every year or few and scrape/sand/repaint the wood that sits on the ground, your best bet is probably a coat of primer and then three or four THIN coats of good exterior grade paint. (NOT thick coats that will get peelier). Of course, cedar or whatever woudl also be good in that case but not everyone is going to spend the money on that for a tractor <g>
Good luck,
Pat