Bare metal shell is spot welded up. As soon as you weld you have rust due to the heat.
Shell is dipped in acid to remove rust
Dipped in rinse tank to get rid of the acid
E Coated - this is a process where an electrical charge is applied to the shell resulting in the deposit of a protective film.
Some manufacturers even rotate the car in the tanks to ensure all air escapes and complete coverage. The problem with Mini's is that it's a 1950's design and has very wide seams. The bottom line is the E coat process doesn't work properly. My guess is they went straight from shell assembly to the coating tank. The result is rust around every spot weld. Add to that a severe lack of paint, box section wax and the result is a lifespan of 10 years.
On the + side, panels are relatively cheap so it's not going to cost mega wonga. I'm not really bothered how long it takes me as long as it's right when it's done