polishing stainless trim

[Greg63]

I have just picked up a set of '63 - '66 stainless side trim for the truck and now I want to clean and polish it . I have read various articles about polishing , but I want advice from anyone who has done it to one of our trucks .
I have practiced on a piece of door trim (lower) that I had laying around . It seems that there is some type of a "coating" that is coming off when I put pressure on it . It is really shiny when the coating comes off , but I fear I may be doing something wrong . You really have to hold it on the wheel and put pressure on it to get this result .
I do not want to screw up the pieces I just got . I am using a 6" bench grinder with sewn cotton wheels and black and green compound .
Can anyone add some advice or direction ?
Greg