What your describing sounds like a problem I had recently-- I was doing post on a poser rendering, and found that two of the smaller figures, only partially seen, had been rendered without anti-aliasing on. It would have been a lot of work to redo them, and the problem was hardly noticable, but here's what I did-- I went around the edges of the characters with the smudge tool, and smoothed out the places where the anti-aliasing was a problem. NOT something you would want to do if you screwed up something major, but it worked. I rarely use the smudge tool, but it has its utility and I plan to experiment with it more. ALL the tools in Pshop are useful for something!