Peter, use a standard etch primer as they bind strongly to the metal and are a very thin coat, which doesn't fill the grain. Use a black primer, as your top coats will be thin and this helps build the color. The best paint for door grain is an acrylic laquer as you lay it down with lots of very light coats and that also stops the grain filling with paint and with doors, you are aiming more for a 'cosmtetic' rather than 'protective' paint job. That means that the paint doesn't go on heavy as you are aiming for good looks.
As everyone has said, preparation is the key and you need to remove all the paint from the door panels as that will dictate what the final look is going to be. If you have any surface rust underneath the paint, then rubbing with scotchbrite will clean it up without damaging the grain.
Brad