Yes, sand it with 80 grit sand paper. You don't have to make it completely raw, just sand off most of it and make sure it's all rough and scratched so the paint can stick.
Then, take your spray and put a LIGHT coat of it. Don't get too close either, stay about 7 inches away from the case and put a light coat. If the paint is running down the inside, you put it way too thick and way too close.
Let it dry fully, and then put another light coat on. Do this 1 - 2 more times and then sand it with 800 grit sand paper, and then do a few more light coats. After that, sand it with really fine sand paper (like 2000 grit) and put some clear-coat on it to protect the finish. You can use semi-gloss if you want to give it a little shine.
Make sure you wait until each coat is fully dried before you do anything.