Does not matter what paint or coating you put on the trailer frame if they prep is not done completely and correctly, because any rust overlooked will spread and ruin your hard work.
If you're going to go to the trouble of stripping the springs and all other items from the frame, you might want to consider sandblasting or glass beading. Not only will these remove all paint, rust, etc, you may find welds that need repair. And the surface of the frame will take primer and paint very well. (I wonder how expensive power coating may cost. Would look great!)