Best solution I've found for rust is to move to the midwest and keep it clean. You get the added benifit of there not being so darn many people!
I spent a year in florida with two of my VW's awhile back, nope, no sir, never again. Every tiny little scratch you couldnt even see started turning orange and crusty. It was a nightmare.
I would like to re-undercoat the 300d at some point.