I don't think you'll have much luck trying to add material to your nylon part. Nylon is very difficult to adhere anything to. Sanding is an inherently imprecise process; it's difficult to get an accurate sliding fit once you've attacked your part with sandpaper. You might consider re-doing your part and machining the critical surfaces rather than sanding them. In that case, printing the positive part a bit oversize (and making the bore a little smaller) will give you some material to remove.

Andrew Werby
www.computersculpture.com