Hi, i tried to prep a glazed ceramic tile with poliurethane varnish but there is no way the cyanotype solution will be absorbed by that. I will now try the formula above. I havent tried it yet because i do not have photographic grade gelatin available.
Just use knox gelatin that you get at the grocery store. It is suitable for subbing. I just coated a glass plate with a knox/gloxal layer and it is perfectly transparent. Good luck!