Wet and Dry Emery paper, preferably used wet, add a drop of washing up liquid to the water is the best method. Moulding lines aren't that big, so start by gently rubbing with a Medium to Fine grit. Finish off with Fine or Very Fine grit to polish the surface.
While this will work with plain ABS and HWABS, any special coatings or finishes like some Marushin and Tanaka realistic blue/black finish may need extra care. Hopefully another member can advise with that.
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Cerwyn
Hobby collector of Replica model guns and Militaria.
also member of Living History Reenactment Groups.