I have done a couple of paint jobs myself for a couple of race bikes I have owned. I learned somewhat and was told by a professional painter to use this
Ultra Cutting Creme
to polish the clear coat. It does miracles. I have ever since used it on all my scratches and dull clear.
I use a buffer with wool fabric and get the best result. You just have to be careful on not over doing it but its pretty hard unless ur really stupid (no offense intended). This is not ur average over the counter stuff, this is professional
You wont find this product at ur regular auto parts shop. You have to go to a paint shop. I buy mine at a Napa store that sells paint.
Now if you intend the lens when you mention left turn signal light, I have had good results with Meguiars Plast-X Cleaner and Polish
Looking around for the Ultra Cutting Cream I came across this thread
http://www.sailnet.com/forums/gear-maintenance/52772-tips-compound-polish-wax.html