@ Kyle - The deadener will help with heat, but I do not recommend it for a primary barrier. I would find a heat shield , or possibly a "ceramic content" paint , to under coat the exhaust area in as a primary means with deadening as a secondary . When off roading as you already know, it allows for alot of heat buildup . Adhesive of most brands are only good to 2-300*F before loss of adhesion or possibility of fire . The general deadener is not water absorbent and is comprised of a Butyl and or Asphalt based adhesive , with a foil constraining layer .
Freemind is correct in saying "70-80% is all that you really need", The idea is to keep the panel from vibrating and it doesn't take 100% coverage to do so .The foam will also absorb sounds entering the car, and sounds that "reflect" ( echo) inside the car thus giving a quieter and clear sounding ride and radio .
Current : 2 15s 149.5@56hz 2k rms Previous Trunk box 1 18 ,146.4@43hz 3500rms,Sony 12s 145.6@63hz