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I would think a good shop will sand them down smooth before they coat them, and from the ones I've seen, you won't notice any previous scratches or cracks



There i beg to differ --How you give them your pipes is how you will get them back.grind down as good as possible,When i did my cast iron ramshorns [sbc]i wish i'd paid attention to the advice i was given, there were places i missed or ignored.But on the genrally smooth parts of them they came out gang busters.
Take the time you won't be sorry jmo

bob s