When I remove rust with chemicals I use phosphoric acid from home depot. For small stuff I mix it 50/50 with water in a big container and let it soak and it works great. For big stuff I use a spray bottle and wrap the part in plastic to keep it wet (as long as it stays wet it active) and let it set. I did my frame this way this is how it looked when done. I emailed to bretcopsey about it a long time ago and I must say it works great. Thanks Bretcopsey just in case I forgot to tell you thanks.
I just hit it with a wire wheel to remove the residue and epoxy and painted.