Check the MSDS - does it have either of the two chemicals listed above?I have Sonax wheel cleaner for my car. I think it works on oxidized metals. Would this be appropriate? Do the other products harm the gel coat?
Any acidic rust remover will clean up the rust. The issue is keeping it from coming back. You will need to remove the grab rail by unbolting it from inside of the hull. This can be a real PITA. After cleaning up and drying the part and the bolts and removing the rust from the fiberglass, you need to make a water-tight bedding seal between the metal and the gelcoat when you replace the part. Whatever you use needs to maintain flexibility. 5200 is not a good choice. I've had the best luck with old fashion butyl tape, which you can get from an RV dealer and with BoatLife Lifeseal. You need to fill up the entire area in contact with the gelcoat or rust will continue to weep out.
Seconding the Barkeepers Friend. It looks like and works like the old fashioned Ajax or more modern Comet powdered cleanser and I use it with an old tooth brush to remove rust stains in cracks and crevices. I keep a container on board the Gulfstream ready to use when I have a moment, waiting for Commodore to come aboard.Bar Keepers Friend works great too, also sold at Home Depot & grocery stores. If its got oxalic acid in it, it'll work on the rust stains.
Also keeps the ants away.