A friend started out practicing on horns that had defects and would never make a nice powder horn. just worked areas of the "bad" horn smooth and through trial and error got really good at it. Never drilled or put plugs in his practice horns.
He told me he spent a lot of time reading and looking at pic's of old horns to get idea's. He also said a lot of the old horns had some small mistakes on them, in his words "it gives the horn a unique, one of a kind, character"
For those who fought for it, Life has a flavor the protected will never know.