I also considered sand blasting, but again a concern is damaging the
critical surfaces, not to mention getting any grains of sand left in oil

passages.

Hi Finn;
        I glassbeaded mine, with good results. Allot of scale came out of them. I covered all critical surfaces with several layers of duct tape which worked fine, then directed then beader nozzle into the passageways. Afterward I blew them out with compressed air. You are right to be concerned about left over grains. I blew & I blew,  from all directions in all passages, (this actually sounds a bit X-rated :-), and when I felt I must have it all, I would get a little more. Finally I got to the point where I could get no more. Then I flushed with solvent for awhile and then air again and found no more, so I'm confident that all passages are clean.

        If I was to do it again, I would plug the oil passageways before beading, as they don't really require any serious cleaning beyond a solvent flush, and concentrate on the coolant passages. Any left over glassbeads/sand in the coolant passages shouldn't be any worse than the scale fragments that you are removing, however with a little effort all should be removed easily enough.

S. Todd Bartrim 
Turbo 13B
RX-9endurance
C-FSTB
http://www3.telus.net/haywire/RV-9/C-FSTB.htm

        "Imagination is more important than knowledge"
                                        -Albert Einstein