Fat Tire products are carbonated at about 2.5 to 2.6 vols of CO2. To maintain this recipe of gas the pressure must be set at 14 PSI @ 38F @ sea level. This pressure should be changed only if the temperature changes or the elevation changes. Using 18 PSIG, your product would be overcarbonated with an off taste that would be more on the gassy, metalic side than flat. Check for gauge accuracy, leaks, correct temperature/pressure, etc. If the set up is correct, no need to ever untap keg and the keg should have a shelf life from the day of packaging of about 55-60 days or until it just does not taste right for you anymore.
Scott Zuhse, Instructor
Micro Matic Dispense Institute