21 day water fast

I have a skin condition known as keratosis pilaris, which is basically a fancy term for ''bumpy skin on upper arms''. But I also have it on my calves. It has always made me feel self concious but I have been researching it recently. Apparently if you Water Fast for 21 - 28 days it will clear up. I am willing to give it a go. However I have only ever fasted for 2 days on water and unfortunately went straight back to my old eating habits. Is it ok for me to fast for such a long time when I haven't had much experience? Is it safe? I plan on eating 100% raw for a week leading up to the fast. Will this be enough preperation? Thanks. Tweet