engraving without scratches. i do not claim to be a expert but some brass items sold for engraving are covered in clear plastic adhesive tape.
i have found when engraving with a center drill or an end mill it is best to leave the film of plastic tape on until after engraving.
1) it helps protect from scratches
2) it helps with burs or rough edges as when tape is removed it helps remove the burrs
i never did diamond drag engraving but maybe diamond will engrave glass with plastic tape on it and remove plastic tape when done engraving ??