Butyl tape was used by many auto manufacturers to seal fenders to body metal. My 1980 BMW 633CSi is one such example. The butyl tape (about 1/16" thick) seals the gap and did not contribut to trapped moisture and thus rust when I pulled a damaged fender that had been in place for 31 years.
One problem that I would see with the butyl tape on the AD fenders is that squeeze out might be a problem. On the BMW, it happened under the hood and was thus not much of an issue. you can see evidence of the squeeze out from the factory fresh installation.
