Which model XTR crankset? I'm assuming it's anodized. You'll have to remove the anodized coating before beginning the polishing process. I've done this several times by stripping, wet sanding with 400, 600 and 1,000 grit sandpaper, followed by a buffing wheel and compound or Blue Magic and a cloth.
This is what I use to remove the anodizing.
I stripped and polished a Truvativ Elita this week actually.
Untitled by
ColonelJLloyd, on Flickr
Untitled by
ColonelJLloyd, on Flickr
Untitled by
ColonelJLloyd, on Flickr
Untitled by
ColonelJLloyd, on Flickr