ive been buying all sorts of tools lately to find the best things to use. Im currently doing an inlet manifold too. Ive blasted the inlet with a 120 grit aliminium oxide grit first to clean it down. I then bought a sanding kit like this
ive sanded it down to the best i can with the pads but they cant get in everywhere but have taken out the roughest surfaces.
Then ive gone into greaseless on felt bobs with a dremel. Like said above its the longest process, you load it onto the felt bobs, then have to let it dry hard ( quicker with hairdryer ) then sand away, when the greaseless has worn off the felt bob ( which it will do fairly quickly ) then just reload your felt bob and carry on. Ive bought a range of felt bob sizes and can get into most places on my inlet now so its just a case of getting on with it.
here's what the greaseless looks like
Hope this helps