Sandblasting will work...go with about a 60 grit sand...but you will need a lot. You need to wear a mask...the silica in the sand is not good the the lungs....not to mention the paint. The other option is sanding.
However, I'm not sure what you will gain. Truth is, if you only have one coat of paint on the metail...and it's not cracking...then it makes a better under coat than bare metal...why strip off something that is already stuck on real good. I would just sand down to the primer. It will also be easier to get out minor dents and other defects. Otherwise, you are looking at a lot of primer and a lot of additional color sanding.