Perhaps I should have put references with my previous post.
Christopher James The book of Alternative Photographic Processes[...]Once the glass is perfect, and dry, you need to pour a warm gelatin, with the Gloxal added to it, over its warmed surface.
And just to be sure I just did a test on a scrap piece of glass with cyanotype chemistry and it came out great.
I scrub the heck out of my glass plates with 'Bar Keepers friend" until I get even sheeting. Once they air dry I warm them with a hair dryer and pour apply 15ml of a 40% glyoxal solution to 250ml gelatin (6.5 grams knox to 250ml cold. Swell for 15mins the bring up to 140f for a few mins).
To improve the prints I'd recommend 2 coats of cyanotype chemistry before exposing.
EDIT: Also I have never found a need in my own experiments for photo-grade gelatin in subbing layer. Perhaps PE can give more information on this