I've used the knife scraping method bu no matter how careful you are a Knife bade is till a straight edge. I bought my self a FLEX-I-File tool a few years ago that work great. It's essentially a 'U' shaped metal frame upon which you attach different grits of sanding strips. By compressing the the top end of the 'U' you allow the sanding strip to bend to the contour of the barrel or other rounded edge. One good tip I read though ,Lightly paint the seam with silver paint before final painting. It will bring out any imperfections in the sanding gluing. :-)

SS-74, Heres what I do, Test fit first and see if the halfs are going to make a good match. Apply liquid glue to both halfs and let it work for a few seconds, put together and tape/clamp with rubber bands. After its dried I apply a little bondo putty and let it dry. I then GENTLY tighten it in the chuck of my Craftsman re-chargable drill and fill a small plastic tub with warm wate.r I then take 600 grit wet/dry sand paper and lay it across my fingers and gently cup the barrel in the paper. While keeping the paper wet, I run the drill and pass the paper back and forth the full length of the barrel. This takes it down to perfectly round. You only neeed to go until the seam is gone. I then finish with 2000 grit to polish it. The rechargable drill runs pretty slow and if you keep the paper wet, you don't have to worry about melting the barrel.HTH