The Merc silver will cover no problem, I got a bit ahead of myself with the advice though, if you were repainting within 10 or so hours then straight over the top would have been OK, however, as its going to be more like 24hrs, then you need to do some very easy prep, while at Halfords, pick up a couple of sheets of Grey Scotchbrite abrasive pads, use this with warm soapy water to lightly key the surface, no need to go mad with this, it will just help a little with the paint binding...
Thom