Looks pretty good to me blue, the key is the rust converter, killing the rust is essential first before painting or ya just wasting ya time!!
Ive been working on my trailer this weekend, taking out a small bit of rust, i used a grunty wire brush i attach to my angle grinder, clean off the scale rust and take it back to bare metal as best i can then apply the rust converter, i used masters "rust.tec" which ya just brush on, it reacts within minutes and kills the rust, wipe down the excess and allow it to dry..
Then i applyed wattyls "white" primer with a brush, 24hrs later wattyls cold galv, looks mint!!
I prefer to use a brush as it goes on alot thicker than the spray cans, you should be alright using 2 coats of primer, not sure what products you can get in black for that purpose!!
Spose you could put the cold gal on then spray paint it black then it would be bullet proof!!

Blue... all my cross members bar the one across the back are closed off... each section has been drilled from the top and a s/s bolt installed.. once removed, each trailer section is partly filled with waste oil which sloshes around as you head for the ramp... this stops/inhibits any rust from the "inside out"... The rear one is simply flushed with water after each use....

RTF... not my idea but a clever one done by the builder of my custom boat...  there are only threaded bolts in the top of each section with a fibre washer to seal... Haven't actually inspected the "slosh" inside.... 

Yeah thats the bizz blue, that'll be sweet mate!!