Since you believe there is the possbility that you may have kept the gelatin solution hot for too long let me further add that you do not want it ever to get over about 140F, not even for a few minutes, and certainly not for several hours. It would probably be ok to keep it warm overnight at a realatively low temperature, say at about 90-100 F, but really hot temperatures will break down the molecular structure quickly.
BTW, since the time of my chapter on carbon in Coming into Focus I have refined slightly my coating method. I now use a frame which I place over the paper to contain the warm gelatin solution and keep it from flowing off the paper. The frame I currently use is of magnetic sign material which i use over a piece of galvanized steel, but plain wooden frames will also work ok. You can see this technique by going to http://rmp.opusis.com/carbon/carbon.html, and I describ eit fully in my carbon manual which is sold through Bostick and Sullivan.