What do I use for a primer/surface preparation so that enamel will stick to the surface of focusing frame. The surface tension - enamel on magnesium is exactly the opposite of what I need.

I'm not exactly sure what you mean by "The surface tension - enamel on magnesium is exactly the opposite of what I need."

I have had no problems at all touching up the metals on the Graphics with a variety of paints, from a small bottle of enamel flat black that I use for lens boards to auto touch-up paint also in small bottles. It all works fine for me...

However, if you're doing a whole back or large portion, a primer may be needed. I would suggest a chromate primer used for painting aluminum. It works great, but is also quite deadly

Maybe you're just having a problem with dirt & greese? A wipe-down with a good solvent to get rid of any greese or oil helps a whole lot...

For light corner touch up I use either Kodak brushing lacquer or Fargo's flat black lacquer.

If the entire panel needs repainting, after thouroughly cleaning off the corrosion and dirt, I use Krylon semi-flat spray. It has proven quite durable and approximates the tone of the original black, although it is not crinkel finish like the factory paint.

I have never used primer on these.