gem, ospho, neutralizers are all just a quick way out that will not last. Ive tried it tons of times with mixed results. Sand it down and apply the right materials. If i were going for a quick and easy way to paint a trailer and wasnt concerned with color i would probably just buy a kit of rustbullet
http://www.rustbullet.com/videos.asp

I used the PPG because i wanted my trailer bright blue!