Must admit I thought the rust was on the inside of the bonnet. That bonnet looks in good nic. Personally I would attack it with one of those 3M easi-strip pads to get all the remaining paint and black undercoat off ( just don't work a spot to long as you can easily generate heat and distort the panel. Used one pad for a whole Escort.) Then attach it with deoxidine - should come up a treat.
Use sanding or flap discs only as a last resort-might be quick now but it will inevitably create work down the track. If you have to use a flap wheel make sure the sanding face is as flat as possible to the work (ie avoid gouges).
Sorry if u already know this and am telling you to suck eggs.