Most upper body rust occurs from accumulations of wet dirt under scuttles, shuts, panel joins, arch rims etc, or ignored and untreated stone chips. Trapped leaves are particularly umleasant as they will give off tannic acid as they brew, I mean decay in the wet. Long term it can also result from the paint finish failing and becoming porous. All these can be cured through regular cleaning.
Underbody and structure is even easier. Buy a straight car, and redneck waxoyl the bejesus out of it. Then every autumn inspect it closely before the winter salt hits.
Factory underbody and structure treatments are imperfect. There's a fine line for a manufacturer to tread between keeping a car structurally rust free for 7 or 8 years, ease of application in a production line environment, and making a profit.
