rust control

I've had limited success with rust control.

Current practice is
1. Cut out rusty metal.
2. Weld in new metal.
3. Spray standard primer on the bare metal.
4. Apply standard body filler.
5. Shape the filler.
6. Spray three or four coats of standard primer.
7. Lightly rub the primer
8. Spray two coats of top coat.

I've been told that I should use a non-porous filler direct to the metalwork (maybe one of those with metal in it?) to get the shape - then finish the shape with normal filler.

I thought a primer direct to bare metal would make the metal waterproof but apparently primer is not water proof?

Perhaps what is happening is that the filler is cracking or I'm not using enough top coat and the water is getting into the porous filler and is rusting the metal, behind the filler.

Bottom line is what is the correct procedure?