the key to good paint is prep work. you shoul dget the paint mixed with the proper flex agents, and use a flexible primer. You should probably clean the side moulding, and sand them to roughen them upto accept the apint. Then you should prime them and wet sand them with finer grits, I.E. 600 and higher.
Once the priming is done and you have a smooth, clean primed surface, you can paint them. When done painting on several even but thin coats of paint, not overdoing it to make it run, and wet sanding in between coats too.
The trick is to wet sand, and use very fine grit, I.E 2000 on the paint, then even higher up to 6000 if you can. After you are finished and have done the final sanding. you should wait for the paint to fully cure, probably about a week, then poilish it with polish and buff them up. then wax them and they should look very good.
I'm sure benjay will agree with me on taking time to prep them. Though he may have some suggestions for the actual priming and paitning methods, I have used this method on y wiper panels, and they came out looking very nice.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.