Well, depending on the severity of the piece in question, I've never had to leave it on for over 15 minutes. The o.p. is looking for a solution to reuse his original trim, and while everyone's ideas would work for most, it sounds like this is slightly beyond just a quick touch with the creme. As a lot of folks know, a buffing wheel can be a godsend if its used properly, if you don't know what you're doing it can destroy what it touches very quickly. Same could be said about the oven cleaner, which is why I threw caution up. I saw gasoline previously posted, that also is an effective method of removing embedded tar.