i have tried to put the emulsion right on the glass as ron mentions can be done
but i have never had any luck doing it that way.
i have used a few different subbing compounds, from varnish and clear polyurethane to gelatin.
the varnish and urethane yellowed after a bit, but the gel never did.
there are a bunch of different kinds of gelatins, and as ron says,
you can use, photograde seems to work the best
( i use food grade which probably isn't good ) ..
after the gelatin is coated and dried ( i do a few coats ) you can either
flow the emulsion on the plate, or paint it. its fun, and not too hard.
the hardest part is cleaning the glass, so water sheets off ( no dry spots ).
there is a great article here: