Take them to a bodyshop
It's really difficult getting a good finish on larger areas with a rattle can, but if you want to have a crack at it, make sure you get some decent plastic primer, tack cloths, Upol acid etch primer, fine wet and dry and lots of patience. And don't do it on a damp day. Wait until it's at least a 20 degree, dry day, well away from dust and dirt.