I agree with EdStainless-- start by observing the polished cross-section with no etching. Use polarization and DIC, as this is very effective for Mg. Heat tinting with fairly low temperatures can also be used. You should only use an acid-based etchant if you want to reveal the microstructure (grain size, grain boundary constituents, second phases, etc.) of the Mg. Is it pure Mg or an alloy? Other suitable etchants involve picric acid or even the standard 2% nital used for examining steel.