Im wonder if anyone knows how to get a professional paint job look by using spray paint on plastic using black paint?

It doesn't have to be perfect. I just to get rid of some deep scratches off the spoiler.

I've done this successfully on several small panels and pieces. block off your spoiler from the rest of your car if it cannot be easily removed completely. Use some 600 -1000 grit sandpaper and sand out the scratches. Go over the entire piece with the sandpaper to provide a good surface for the paint to adhere to. Next get a good quality paint, clearcoat, and adhesion promoter from an auto store or on-line merchant. Apply the promoter to the work piece and, after it is dry, strt applying paint in several thin coats so it does not run. I typically use about 4-5 LIGHT coats letting the paint dry between coats by about 30 minutes. Once happy with the color coverage, let the panel dry overnight before applying the clear coat the same way you did the paint. 3 LIGHT coats should do it. After allowing the clear to dry, it can be wet sanded with 1000 -2000 grit sand paper to bring out the shine and get rid of any imperfections. Wait about a week or more before applying any wax or sealer.