Yes I think you are expecting to much from a spray can.In a spray can there is only about 30% paint and the rest is thinners,so there's not really a lot of paint.To get the hardness your after you would really need a compressor and use a 2pak paint.The easiest of which is a COB paint(Clear Over Base).When the clear is put on a harden is added it will dry rock hard,but in saying that it still can chip.Sounds to me like your looking to powder-coat them.It's a powder that is sprinkled on then cooked and basically it becomes a plastic coating over the metal very hard and very hard to chip.
