I'm the guy that posted that link but I didn't do it to cause controversy. Perhaps we're reading this wrong and maybe the word "charring" has nothing to do with approaching the flash point of the material. Several people have used spray on foam on their tanks with much success and would have found darkening, cracks or degradation by now. In my own case, I attached Isocyanurate panels to my tank and actually used Great Stuff as the adhesive and have found no problems. Of course mine is unpressurized and routinely only reaches 185 to 190 at the top. To feel more secure you could purchase a DIY kit of high temp. foam, the smallest number of board feet that they offer and apply a thin coat to the tank and then call in your foam application contractor to finish it to the full thickness desired. Do I think you need to do this? No.