The 180 degree columns or 1/2 circles that you put together are somewhat recessed but not much, however if they are stacked you still have that to contend with because they are not recessed on the ends. I bent a 8" & 10" knife to finish the verticals but still ended up rounding them out even more by troweling horizontal so they wouldn't look egg shaped or oblong with bulges. No matter how you do it it is very time consuming to make them look perfect. I did as you mentioned with sand paper buying a roll and using it to buff around the column. Then I sponged it with a damp sponge (to eliminate any sanding marks) because the composite is smooth as glass.I think the diameter of the column I did was around 20 inches but Silver could offer a more experienced suggestion.
One thing I would definitely do is recess those seams and then have a rounded knife made to conform to 1/4 of the circumference of the column (so it's an easy "up and down" finishing process). If you rely on straight knives, they will round the seam horizontally and it will get wavy...and take too long to finish. Any metal shop could form the "knife" for you. It would cost almost nothing. Some long strips of sandpaper (24 inches or so) would be nice so they could sand it all down like shining a shoe. That would be a great kit.
My $0.02 :thumbsup: