Keep sanding and applying the SEM problem plastic repair until you get this effect. You will still need to use the paint stick with stick on 80 grit to get your lines crisp.
Here are a few pictures of how it should look like when sanded correctly, I still need to do some detail sanding and smoothing the body lines of the spoiler but you get the idea. Also there is no SEM no swell body panel plastic repair on this part yet.
Once you have achieved the desired lines, apply the SEM no swell plastic body panel repair glaze. Do not use Bondo because it will not flex like the SEM product is designed to.
Spread this on after you have sanded the spoiler area down with 80 grit and a DA.
You will need to apply a few coats, it spreads just like bondo.
Sand inbetween coats of this stuff and final sand with 600 wet sand paper then prime.