I was thinking of solid nylon; evidently SLS-printed nylon is a lot more porous, so that even though paint won't actually stick to it, it will build up in the pores. But I still doubt that this built-up paint layer will give you the smooth sliding fit you're talking about.

You say it's too complex to fit in a lathe chuck, but that's a common problem in machining. You can use a fixturing compound to surround the complex end of your part and make it grippable by a chuck; these typically soften when placed in hot water and can be removed. Jettset is a plastic one, there's also Cerrabend which is a metal.

You didn't show us the part you're making, but there are other machining tools besides lathes. If it's complex, you can probably make it or refine it using a CNC mill with a rotary 4th axis.

Your sanding technique sounds fairly controlled, but you may have made your interior piece too small to start with; if you had more material to remove it might have worked better.

Andrew Werby
www.computersculpture.com