I used Chassis Saver. It has the same caution. My plow will not be left out in the sun. I don't doubt the effects of UV, but I think the topcoat suggestion is largely CYA advice. A local welding/hydraulic shop made a stump remover for his skid loader and used Chassis Saver on it. He leaves it outside year round. The finish is still in good shape after three years with what one might call just a little chalking on the surface. I have seen industrial enamel look worse in the same time.
My main concern is adhesion to the base metal. As for top coating, the cured surface is so glossy (I used gloss) that I wonder what would stick. As an aside, I used to use PPG DP 40/401 epoxy (and well as its other colors). It stuck to everything and everything stuck to it. You barely needed to wipe the mill oil off of steel tubing, and it still stuck. Of course, it is no longer available because of lead. Last year, I tried Dupont 1858S enamel primer on a sprayer frame. I liked the way it applied, and its acrylic enamel topcoat (just regular automotive chassis enamel) has stuck well. It was on the recommendation of the aforementioned welder that I tried Chassis Saver for this project. Next Summer, I hope to have my questions answered. For projects where appearance matters to me, I do like to spray. I think Chassis Saver would be a real PITA to spray, to say nothing of what it would do the the equipment. I did use a mohair roller on the large flat plate of my attachment, and it seemed to work OK.
John