Did all underneath with waxoyl and did all the cavities, behind all the wheel arch covers, bonnet, doors, hatch internally the lot.
Why? Read the many rust threads on here for a start.
Other reason? because of the lack of protection except a few plastic covers.
Another reason? I did my mk1 GTI back in 1990 with a few topups since and it seems to have worked on that.
Cars use to be undersealed, sadly there like fridges now, just simply white goods most people just throw away after a few years and the manufacturers build accordingly.