Many years ago I was told by several bodymen never to put body filler over bare metal.always prime or seal first.

I learned the hard way in 1961 about Bondo over bare metal when I repaired a small hole behind the rear wheels on my 56 Ford Victoria. In the 1980s I had a part time job 
writing estimates for Maaco, people would do their own body work putting filler over bare metal and have us paint the car. Six months later they be back with a car all rust 
saying we screwed up. (We always put on the sales contract that we didn't do the body work and that the rust would come back.)

Being stuck at home I've had a chance to watch most of the custom builders do great metal work but then put body filler over their work, where the restoration shops
always seal or prime first.

What is it that I'm missing, are there now fillers that don't wick water?