Wash the surface with TSP and be sure to rince away all residue. Then sand thoroughly with 150 grit sandpaper, be sure you degloss the surface. Then remove the dust with a tack cloth, and apply a coat of XIM Primer and allow to dry for at least 24 hours.
XIM is an ALKYD "adhesion type" primer and can be tricky to use. There is a reducer that you can buy to make it a little easier, but the more you thin it the easier it is to get drips and sags. DO NOT thin it with anything but the reducer.
Once this stuff is dry, sand it lightly with 220 grit sand paper, clean up the dust with a tack cloth and your ready to paint.