Hi there!
It's very difficult to get paint to stay on countertops for an extended period of time, especially if it's a high use area like a kitchen or bath. I try to steer my customers clear of doing this unless it's for a quick fix or a temporary dress up until they get new countertops.
If you do have to do it, clean it very very well and use a bonding primer like Bin or Kilz, Cover Stain or anything oil based that can be applied to "glossy" or "hard" surfaces. I like a good quality oil based finish if you can deal with a little odor and a slow drying paint, but imo it's well worth it.
I like oils for anything furniture-like and floor-like. Oils just level better and dry to a harder finish faster, where as a latex (water based) will stay soft for quite a while until it cures, which can be upwards to a month and show more brushmarks. (however I recommend latex for almost everything else)
Stay away from the poly! Paints can protect themselves just fine (well, one's with a sheen anyways) and a poly will only yellow over time and distort your color.
Hope this helps, I'm not really a know-it-all, I just run a paint store.
