I think I saw somewhere an inflatable sander that could flex around
and inside curves, may have been on the end of a drill but I think it
had an air supply continuously pressurizing the system. Anyone know
what this contraption is called or how it runs?

If not, how best to sand a load of wooden octagonal rails, 6" apart? I
want to smooth them over after presure washing, to take the splinters
out and make it smooth.