Should I put a coat of clear gloss medium on before the glaze?

We suggest applying a clear gloss medium as an isolation layer. This will protect the surface underneath, and give you a uniform surface to work on. If you have some precious underpainting that you would like to preserve - use a clear gel medium as an isolation layer to retain some of those nice areas. When the glaze is newly applied overtop a clear medium - and if you're not loving it - then you can go to the sink and quickly wash it off since it is still fresh. Once the glaze is dried, you have to sand, Scotch-Brite or steel wool it off. Here is where you should be somewhat discerning and decide what parts you want to expose and what parts you want to leave glazed.