I find flap discs tend to polish the rust, an angle grinder with one of them wicked wire bush attachments is bloody good, decent gloves and face covering essential.
Bottom left and top right versions are fast and evil!
+1 on the protective gear if you use these, I did when I stripped my chassis, thought I was alright with gloves and one of those full face masks that are used when heavy duty strimming. Had a sharp pain in my leg and one of the wires had come off and went through a pair of overalls and a pair of jeans to stick in my leg, not impressed. May be safer if used in a drill (corded), angle grinders are rather quick.