A subbing layer is not normally needed, but one goes like this.
10% gelatin 100 grams
4 ml 5% chrome alum stock solution (stock is 10% chrome alum)
5 ml Everclear (95% ethyl alcohol)
1 drop 10% thymol in iso-propyl alcohol.
This should give a good subbing layer for you coated using normal plate methods.
If it is too thick, just dilute with distilled water in steps until the desired viscosity is reached.