say it with me, "preparation is paramount". at a minimum, you are foolish to not clean that metal thoroughly like I described above. nothing, and I mean NOTHING will reduce the life of a paint job more than putting paint (yes, even badass catalyzed epoxy or industrial enamel) on top of dirt, mildew, algae, grease, etc. I'd bet that all of that and more can be found on your trailer right now. at a minimum, clean it, address the loose rust and paint it by hand from a lidded can. if you want to spray it, use real paint and an adequate sprayer for the material. I would not bother spraying as you will do a better job by hand.
aerosol cans of paint have their place, but are diluted with solvents and propellants considerably. you need to apply 6 coats from a rattle can to achieve the mil thickness you get with one coat applied with a brush or roller. I bet you I can paint that whole trailer with a 4" roller and a brush twice in 3-4 hours once the prep work is done. you don't have to break the bank or spend a week to make that trailer look good and stay that way for a while. maybe converting the rust is going overboard, but it is meaningful step that will keep the rust you treat from returning. use a garden sprayer for this if you bother. makes coating that expanded metal pretty fast and easy.
if minimum effort is desirable, get a couple of cans of Rustoleum and shoot the rusty parts a couple of times. it will look better from across the street and will slow the rust down a little bit.
This is the approach I'm going to go with I think. It's a $700 utility trailer... I knew what it was when I bought it. But I'm not going to go through the hassle of painting it just to watch it rust again in 6 months. Just because it's a cheap trailer doesn't mean I shouldn't invest some time and money to help it last longer.