I am building a 1973 240z show car so everything is going down to metal including underside. The fastest way is to media blast but their are dangers in that the wrong media can damage the metal especailly thin areas, the safest media is made of plastic (of course it costs more). The other problem with blasting is it leaves debris in every nook and cranny and it sometimes never goes away. I prefer to use an enviromentally friendly product called Ready STrip which will peel off the layers of old paint (5 to 7 coats worth). You apply with brush or undercoat sprayer, ket sit for up to 24 hours and then pressure wash off. Any paint that did not get removed can me given a second coat or sanded down with the appropriate grit. Very important, once down to metal the car body will begin to rust, so get it primered as soon as possible. For best show quality and longetivity avoid using bondo or other body fillers, whenver possible. Remove the dents and body damage by manipulating the metal, this is why some body guys are called Artists and not body guys. Artist's have years of experience and can move the metal the way the want, body guys throw a coat of bondo down as fast as possible and move on to the next car on the assembly line.