my friend has his valences painted and no they didn't smooth them properly they just did a light scrap and then sprayed them. he's had them painted for about 2 years now and they look really good. as long as u get them to do it properly the paint won't peel!
sanding before is a must if you want it to look it's best, i have heard a lot of people using a 300/320 grit to scuff. i have also heard of people stageing it, strarting with like a 60 grit, and moving up to 200-300-400 saying that any heavy scratches will get filled anyway. as far as paint, it is a must to have a flex agent in the primer, paint, and clear coat. there is some decent info here: http://www.paintscratch.com/automotive-paint-tips.htm