I chose to remove the paint by hand (razor blade) on the exterior as sand blasting builds up too much heat and causes warpage. With respect to the undergear, I did do sandblasting in stages, when I was replacing or repairing an area and then I sand blasted the undergear again after it was all completed. Before I did anything, I put in a set of subframe connectors that help keep the car straight during the process. After sand blasting the underside, I soaked everything with a marine based primer called Interzinc52. That stuff is so tough that it barely scratches with 80 grit sand paper.
While my car is no classic, I believe the process works. The full story is at my cardomain site.
http://www.cardomain.com/ride/2219901
/paul