Fixing fenders? Rust protection?

[Bubble_Drift]

I am going to fix my fenders soon. I was wondering what is the right way of doing this?

First sand off all paint and bits of rust.
But do I need to treat the bare metal with primer or something else before putting polyester putty on there?

Then primer over the polyester putty, paint and done right?

Here is a pic of the fender:

[LarsZ]

Dont use primer before using putty, just add putty to the bare metal, then primer and paint