We roll the car outside and give the whole thing a thorough pressure-washing with lots of detergent, especially those nooks and crannies known to harbor dirt: the under-fenders, inner bulkheads and the whole engine bay. Then the whole thing gets blown off with compressed air to remove the water and to clean out the aforementioned nooks and crannies.
Assuming that we have made the decision that there are too many coats of paint on the car, or that the paint is not in suitable condition to serve as a base for an additional paint job, we start removal of the old paint. Generally this goes quickly, with a coarse-grit sanding disc on a dual action sander, though at times chemical stripper is employed. We are looking to get down to good factory paint and primer. Once we get close, any corrosion will become evident, as the primer will have started lifting off of the corrosion below. I am perfectly happy to leave good factory primer on, as long as it is not adjacent to corrosion or rot I will be doing surgery to remove.
The DA sander gives way to hand sanding in the corners and edges. At the end of the stripping we will have bare metal at the rot areas and rough-sanded primer and paint over the rest.