Well the top side was chipped. All you can do is touch up spots like that. I normally just use a little artist brush on places like that. The underside looks like all the rust was not removed and the primer couldn't make a good bond. If you could take the deck plate off I would sandblast it again before it rusts any further and do it over and paint it in the same manor.
Urethane is a good finish for a tractor because it will resist all the oil and chemicals a tractor gets on the paint.