On Sunday, February 12, 2017 at 3:01:15 PM UTC-5, Clare wrote:
On Sun, 12 Feb 2017 10:43:40 -0800 (PST), Christopher Tidy
wrote:

Hi folks,

Quick question. Many cars from the 1980s used to rust badly. Sometime in the 1990s, this changed - and quite suddenly. Does anyone know what specific changes were made to the paint composition and surface treatment? I can only find vague allusions in most articles.

Thanks,

Chris
Big thing was change to the surface preparation of the steel to
either electrostatic wet application or full body dip of a high
quality etching primer which I believe contains Zinc.
This was combined with the use of high strength steels and
"galvanized" steel in rust prone areas.

Right. It's better coatings, better primers and protection, and better application. Read, water-based coatings that often are based on urethanes; phosphate and weldable, etching primers; galvanizing in rust-prone areas; and electrophoresis and electrostatic application. The first water-based coatings -- used into the '80s by some manufacturers -- had poor adhesion and didn't weather well. They're MUCH better now.

All of this became more necessary as body panels got thinner, with the use of AHSS (advanced high-strength steels; a continuing evolution of the HSLA [high-strength, low-alloy] steels that were first used in the '70s). Rust is potentially a bigger problem than ever because the steel is thinner.

--
Ed Huntress