Most rust starts on the INSIDE where all the moisture and crap gets up inside and does its dirty deed and only after its erupting through the skin can you see it and then the real damage is done. Internal cavity wax inside all the nooks and crannies and lots of it does the most good IMO. Undercoating the externals also but getting inside with the protection is just as important and often overlooked.
http://3mcollision.com/3m-rust-fighter-i-08892.htmleastwood sells their own flavor in quarts and aerosols as well.
Our 1999 Beetle is a pile of junk however VW puts the rest to SHAME on the rustproofing that car got from the factory. From the floor up the side of the car about half way every internal/hidden/boxed structure is coated with a thick layer of cavity wax. On a super hot day with the windows up it actually smells like melted crayons for a moment when you open the door.
