Ideally on plastic you would do such a thing. You can simply scrub it with a scotch brite pad to rough it up while you are cleaning it with acetone or MEK and get it pretty clean and this will help the paint stick to it. For the professional types of jobs it would be a media blasting a couple of times with something like a 60 grit oxide. Plastic is made out of oil, and it will leach out oil again with some time so all the cleaning and prep in the world can't prevent this.
Krylon is not going to be the longest lasting paint job in the world, but does pretty well and is very cheap to do any touching up if required. It is pretty difficult to get any sort of paint to stick for a long time to items such as the Butler Creek Flip Covers that go onto the scopes. You will notice that I skipped painting the rubberized grips on the AR15 as it won't last too long there with the acidic sweat and general wear from your hands as you use the gun. A clear finish helps quite a bit as you first have to wear through or scratch through the clear to get to the color.
There are many other gun finishing products that one can use that outlast Krylon a good amount of time. I use DuraCoat quite a bit, but I wanted to write this one using a method that about anybody can do and at a price that about anybody can afford. I just completed one of the DuraCoat jobs on my new coyote rifle and it is shown below.