Painting porcelain is like painting on glass. The surface is slick and non-porous so paint has little to grab on to.
If you sand the porcelain to remove every bit of gloss to give some "teeth" to the surface, you will have decent results. Sandpaper grades of 320 or rougher are best. Some scratches with some gloss showing through won't cut it. I recommend a quality automotive primer surfacer that is activated with a hardener for good adhesion. Follow the directions for the product, especially safety and respirator requirements.
That being said, I encourage you to consider polishing the porcelain and touching up spots that really need it. Original porcelain gas pumps have a strong appeal for a lot of collectors.
------------------
Jim "Oldgas" Potts
Your host and moderator