I would have the existing coating removed and re-undercoat with a liquid coating. Something like Fluid Film, Woolwax, or Krown. Any of those coatings costs around $150 per application and should be touched up every year. They will creep into the rust and stop it on contact, you don't even need to grind the rust off. They also never dry, so they are self healing, and water can't become trapped behind them (they isolate the metal from air and water completely). I have a combination of Woolwax and Fluid Film on my GX (DIY application with a spray gun and compressor) and have been very happy with it, but Krown is also supposed to be good (it is oil based while the others are lanolin based) and there are lots of shops in the NE who apply Krown.