Shellac is used to seal raw wood to either avoiding blotching when staining or used to seal oily or tannin rich woods so you can topcoat. For example, polyurethane won't cure if put directly on rosewood. Sealing with shellac allows you to bridge from the raw wood to be able to topcoat with polyurethane.I'm not a woodworker by any means, why the Shellac seal coat?? I thought Shellac was mainly used to seal in odors.