I'm trying to touch up this part ?

So I touch up a few things around my car but there is a part in my roof that the paint looks like a bird poop on it and it was left for the sun to destroyed the original paint lol . I try adding the touch up paint I purchase from bmw but the surface starts too look worse when I'm just adding the paint on top . Do I have to used wet-sand paper first then add the paint or do I just need to add paint little by little . I added a picture , you can't really see it due to the light but the area is towards the right side around the middle of the picture . Thanks for you help in advance .