I build model airplanes and have, on occasion, had to perform the same procedure on the 1st former of the fuselage where the noseblock sets.

Getting it flat for a good joint is only one issue. Equally as important is getting the thrust line true. Just because it's flat, and all gaps are removed, does not mean the driver will be pointing in the right direction. I'm not sure how critical that is with horns. It's way critical for propellers!