In this YouTube video, Quinn Jacobson demonstrates the proper technique for protecting a collodion emulsion with a varnish consisting of gum sandarac, alcohol and oil of lavender.  If you watched the Sally Mann video (two posts back), you saw her scraping a collodion emulsion from one of her glass plates in the field.  Perhaps an initial pour did not go well and she wanted to recoat the plate; or she was not completely satisfied with an image and wanted to start over.  Collodion can be easily removed from an unvarnished plate.  Left unprotected, the emulsion will eventually begin to peel off and may completely disintegrate.  Varnishing is simply a way of ensuring the archival quality of the plate.