Agree totally Dick. The gyprock stuff goes hard as well, so would end up with the same problem. I have been using the sikaflex for well over 10 years in every vehicle I have done. The standard 11FC is good enough, but if you use the marine grade it's even better and can be painted over with any auto paints and also great bonding strength. It stays flexible as well even under the paint where as pointed out earlier, seam sealer hardens. The only thing the 11FC doesn't like is chlorine, so don't drive your car into a swimming pool. I reckon the soda blast guy might be the same guy that recommends putting eggs in the radiator to stop leaks. The 3M stuff should do the same job, however I haven't used myself