John I use 96% and i leave the shell in for 10 to 15 min then check it and put it back in for more time if it needs if . the longest i soaked a shell was a Bachmann 50 ft reefer that I forgot I put it in the shell,(the timer will run for 30 min max.) was in the tank for over 2 weeks it was ok and there was not a lick of paint left on it t.