i might have misunderstood what your problem is, and after reading the other responses
i now realize you are making SG tintypes / ambrotypes ...
if that is the case musila is right-on ( as artonpaper said ) ...
the good folks at RC also suggest using their AG+ emulsion because it has a higher silver content
and is better ( i guess ? ) at doing this sort of thing. i too am starting to make these sort of images
and i was thinking of doing exactly as you are doing, on glass so i can at least see if i am making an exposure
that is working ... on the black metal i have found it nearly impossible to see what i am doing ...
regarding their developer ... with thiocyanate and fix in the bath, it exhausts faster than a conventional developer
... so if you are making a bunch of things you might want to divide up your developer so you can use fresh from time to time
i have never had trouble subbing with gelatin, the only times i have had trouble was with minwax urethane
because it yellowed a while after it was applied. maybe the yellowed minwax urethane would work well
as the yellow toner artonpaper mentioned
good luck !