Is the grain scratches on the plastic or discolouration of the plastic. If it's scratches, you need to sand the surface with some finer grade sand paper until the scratches are less obvious. If it's discolouration, You'll need more coats.
Another problem could be the kind of paint you are using. Some auto paints can 'craze' the plastic. I tried a gloss black auto enamel on some plastic the other day and it dried really grainy and flat. Almost like it was trying to peel. I'd say this is more likely to be the problem. If you have a spare piece of plastic, make sure it is smooth and spray some of the paint on it. If it dries "grainy", then that is the problem.
Hope this helps.