Hi Graeme
If the touch up areas are easy to sand, clean them up with 600 wet and dry, used wet with a few drops of wash up liquid, sand and feather the edges, then go over the whole hull lightly just to key the surface.
At this point i ask if the chips of paint have been knocked off or the paint is not sticking to the moulded hull in larger areas?
If all ok spray with a rattle can of grey acrylic under coat. I use Simonize but if only doing a small hull like yours try the smaller cans from the pound shops.
Always remember, preparation is all. You can't cover cracked or split paint, sand it smooth.
Regards
Mark