Fish gelatin cannot chill set. It is therefore a thin runny mass at temperatures much around room temps and even below.
If you coat it to use as a relief image, you must remember that it should be exposed through the base to allow adhesion to the support, and since it is not chill set, it causes severe problems in manipulation and handling during exposure.
It is not widely used in photography due to a number of problems. There are extensive publications (patents) on this granted to Kodak and a number of other companies.