Most of these paint ons work with phosphoric acid in them turning the rust (iron oxide) to iron phosphate which is an inert black compound. You wash it down after and when dry, paint.
Now you have a chance that way but what works for me in cattle yards crushes and crates is get as much rust off as you can by any means -coarse 30 grit pads on a 5 inch grinder works best for me. Get 85% pure phosphoric acid from a chemical coy and dilute it with an equal quantity of water. It should warm up slightly when the water is added. Paint this on to your cleaned surface and let it work for at least an hour. Yes it will eat galvanising or rather it cleans it pretty good so keep it off the wire .
Then wash it off with water and a rag ,it will be slippery at first. Let it dry and you will probably see some white powder on drying ,dust the powder off and wash it again.
When dry paint with an etch primer for metal -I use wattle etch primer and if you got it on the wire paint the wire too.
Then paint the post ,within 48 hours I am told recently but I am not sure that within 48 hours is necessary yet. For the best results use polyurethane paint I have found ,not epoxy ! Even the old oil base paint with turps cleanup is better than epoxy . The sun destroys epoxy quickly.