The one in my wagon was pretty bad, so I just used a broken off drill bit in the air drill and cleaned off all the rust, then used some rust converter, left it for a day, then fibreglassed over it. It was pretty hammered.
Here's some pics just after I applied the 2nd coat of rust converter. As u can see if was fairly bad.
Here's a cowl panel I chopped out of a wreck b4 we got rid of it, sold it to a guy on here. As u can see, then a fair few spot welds which need to be drilled out along the back, and the sides I just chopped out with an air hacksaw.