I may not be a very smart man, but I grew up using water as a shellac retarder. That's what Zinser uses as well. I remember posting something about the similarity between the chemistry of alcohol and water (boiling points etc.) Depending on temp and relative humidity the drying time is extended from 2 minutes to as much as 12 hours (think August) The thing about extended shellac that I don't like is that it is pretty gummy. A common product, sanding sealer, is an extended shellac and is difficult to sand for a long time after application. It's basically an overnight product. A dewaxed blonde shellac is an over-coffee break product by comparison when mixed with anhydrous ethyl alcohol. Soup come to nuts I'd wipe the pieces with old cotton t-shirts with a 2 lb cut.
Is it a larger molecule? Yes. Water is miscible in alcohol. That is, they mix in all proportions. Put water in shellac and the water bonds covalently to the hydroxy group of alcohol. Heat will break this bond, releasing the water molecule rather quickly otherwise you are relying on evaporation, hence the retarder effect. This is why it is important to use anhydrous, or water free, alcohol when mixing shellac for the french polishing technique. Any water present in the shellac makes the polish sticky and "draw" leaving the characteristic fog of a bad polish. It is commonly resolved by using a finishing pad of (anhydrous) alcohol alone. If you are brushing shellac use a 1 lb cut and sand with 320 between coats. Assemble and apply a slightly heavier brush coat (2 lb is fine) with a fine nylon brush (a golden something something,Taclon?). For all intents and purposes this will give you a fine finish.
