I need some help folks...

I've got my fender close to bare metal and there are a few highs and lows that I'm going to fix with body filler. Here's my question, Do I need to lay down something before I add the body filler? Anything to prevent rust? I've only layed body filler once before so I'm also interested in what everyone uses. Thanks again

 

Today, most shops use epoxy primer first.

Years ago, filler was used over cleaned metal.