Paint removal
Removing paint on a car is at tedious job, specially if you aim at removing all the paint in order to do some sheet metal work or the paint is in such a condition that it all have to come off. I usually prefer to remove all paint in order to find old shoddy repairs that are done only with filler. Areas with thick layers of filler are a source of troubles. The “bondoe” will sooner or later start to crack and loose its grip onto the sheet metal if it was applied in a to thick layer. The maximum layer thickness to apply is about 5 mm. (If you need more than 5 mm of filler you haven’t done your sheet metal work well enough).
You can find areas with thick filler by using a magnet.
For removing the paint there are several methods, such as blasting with different materials such as: sand, glass, dry ice, soda (baking powder) or even walnut shells. These methods are expensive and usually require that the car is taken apart completely. Blasting with soda is the most gentle method since it does not “attack” the metal itself. 
It will even have some trouble removing rust but the discoloring in the metal at the welding remains intact. Such is the gentle touch of soda blasting.
When it comes to blasting with sand, glass or walnut shells, the method takes off rust but it is much more aggressive to the metal surface. There is a high risk of dents in your sheet metal parts. What happens when blasting with these materials is that the material hits the metal surface like millions of small hammer blows, witch will expand the surface of the metal. So to make sure that your fender or door is not damaged by this work, it must be possible to blast on both sides of the sheet metal so that the expansion is done on both sides.
So, is blasting a bad idea? Well for my cases it have only proven to be a good solution on areas that are not likely to loose their shape due to the form they have. Some good examples are the underside and floor of the car. Door frames/edges and generally all edges on the car body, like door /window openings. These areas are perfect for blasting and makes the paint removal job much easier.
How about the large areas you might ask. In many cases I have found that a heat gun and a narrow scarper/spatula are the perfect tool for removing paint. Heat up the paint to the point where it gets soft and then just scrape off. If you are lucky you´ll get all the paint off in one go and what you are left with is the original metal surface that was created at the factory. BEWARE! Do this in a well ventilated area or wear protection, since there might be some fumes. Also don´t heat the surface to much so that the paint caches fire or you get discoloration of the metal. If the metal changes color there is a risk of creating dents. As the metal start to heat up it will buckle but it will go back to normal when the temperature is back to normal. Let the metal cool down completely by it self and sand lightly thereafter. Work done...
If you do not feel secure with the heat gun method, the only way left is with an oscillating sander and a lot of patience.