How would you make hide glue form scratch? Is this true-Adding alum or skim milk will make hide glue waterproof, other techniques work to do the same thing.]. I was also wondering if fish air bladder glue was waterproof.

One method is to put rawhide and sinew scraps in a crock pot covered with water put the control on low and check it in the morning. You will have to skim off the crud. When it is the consistancy of syrup pour it our on a sheet pan linew with wax paper and let it set. You can cut it into chunks, bag it and freeze it until you need it or allow it to totally dry and grind into powder.