I used some xtroll from jota marine. Took a few coats of the rust conqueror first then some silver. Not cheap but fairly happy with the results so far. The other option is to get it re hor dipped galvanised which is not as much as most people think if you ate prepared to stip it down first.

On mine. Used wire brush in angle grinder. Used metal shield primer. My trailer galv was starting to bubble ,and rust was starting in some areas. Very impressed with product. Water just beads of it now been done for good 4 months now no signs of anything coming back. I put on with hand brush looks like is was sprayed. Used same product for springs and axels ( black), make sure you use p2 mask when cleaning trailer

I just bought a 2 lt tin of Zinc from bunnings and a few cheap brushes.

I had my axles etc of at the time as i was fitting new break and springs.

Hit it with the Karcha first...High Pressure fresh water wash as such....then went over the worse areas with a wire buff wheel on the grinder to remove any heavy scale.....water wash again....then slapped on the zinc.

If i had the money id strip and get it dipped in Galv again.

 by the time u grinded fkd around with 3 layers of paint- which if u lucky is gonna last 2 years

comapre price of paint and time lost etc. every 2 -3 years

 take everything u can off

go to  hartways galvanisers - naval base, cut the u bolts and take of towhitch and lights

leave the  axles and wheels in the yard - pick it up a week later- new u bolts on and towhitch back on( if u ask niccely the blokes will even forklift the frame back onto the axles) - u got a "brand new" trailer good for 10 years again

 if u speak to them u can get under $2 a kilo