Say you were throwing a party.. All you had was a fire hose, 20 pounds of jello, a cattle prod, a carrot that only sings songs from the 80's, bongo drums, and a pool full of whiskey.Who would you invite, and what would you do??
car wash!! Seriously. pressure washing it will help. And about the length-you fold it in half. i have one of those.
Most home extinguishers simply use a compressed gas (often nitrogen) to expel a fire retardant chemical. It's impossible to say if the tank is outdated, but firing it will reduce the pressure in the tank. It should probably be inspected and serviced or replaced to guarantee effectiveness. The chemicals are generally safe, but I wouldn't want to breathe them in or have it on my skin more than necessary.
