Good luck sanding the sound deadening, that would be an absolute bitch of a job. If you want to remove it all and start again then get yourself a wire brush wheel that you can put in a drill from bunnings etc, cost like $5 each so get 3 or 4 to do the whole lot.
You'll be amazed at how quick you'll carve through it. :)
Also read the can as to weather you should prime the bare metal first. You should be using etch primer for anything bare metal anyway.