I found the instructions for that product, looks great.

 http://www.permatex.com/documents/tds/Automotive/81775.pdf 

Brown County Mini

I have never repaired a door skin.  I hope some one else can weigh in here.

You will need to sand with 80-120 grit sandpaper until the metal is shiny again and all the rust is gone.  average rule of thumb is if you have a nickel size rust spot sand the spot until it opens up to a silver dollar sized open spot of clean metal.  If the metal is rusted all the way through, you will need to cut out the old metal make a patch/replace the panel.  (Side note:  If you have an angel grinder with a steal wire brush head it will make short work of any problem paint/rust spots).

There are some produces that will oxidize the rust but unfortunately rust is not an easy fix, if you paint over rust you will only compound the problem giving the the rust a good place to grow. Tar and undercoating are about the only things that will stop rust. but those are not permanent fixes. eventually the metal will weaken.

(Insert snide comment here)    

Cracked paint with rust showing thru, or cracked metal?  Cracked paint with rust underneath should be handled as described.  Cracked metal should be weld repaired, then ground flush, and then treated like a bodywork repair.

SE7EN

I believe is called door skin, it is separating on the top right corner, near the handle.

How much do I need to sand the tiny crack? until there is not rust?

There is not any product that stop the rust? or if I paint on top it will keep rusting inside? 

The best way is to sand the area, re-primer and re-paint.  Picture rust like car cancer, you need to cut it all out before the problem is gone.

I don't understand what you mean about the door. is the molding separating?

(Insert snide comment here)