Don't use any of that paint over rust stuff....regardless of brand or marketing. If you have somewhere you can't get clean, a better alternative is to treat the area with OSPHO, neutralize, next day SPI epoxy. This will be a longer/better solution. I have some done this way (before I started using SPI epoxy) 20 or more years with OSPHO and UTECH Epoxy (SPI is heads and shoulders above that). Short of blasting to good grey metal or cutting out and replacing, this is the best alternative method.