Awning cleaning and reproffing

I have an Isabella awning circa 1990. Waterproof and in excellent condition BUT its very dirty. Cleaning products say that you must waterproof after you have cleaned. Trouble is my awning has never leaked and Im wary about 'deproofing' it just for the sake of getting it clean. Any advice would be welcome. (After washing and then reproofing my outdoor jackets etc they are never as waterproof as they were before - and I always use Nikwax).