I found Water Fasting diminshed a number of my scars. I had a few hot plate/oven burns on the hands where one scar is completely gone and the other has faded significantly.
I had a several surgical scars that were keloid like and raised. They are now flattened and the sutures marks look like mini tatoos. They are faded as well.
I had some Acne marks that completely disappeared. Roscea, which presents as a ruddiness to the complexion completely disappeared during the first fast and toward the end of the second fast. It is much better than it has been even under the treatment of a dermatologist and avoidance of triggers.
Lastly, had a tear in my median thigh from playing as a child climbing a barb wire fence to visit a neighbor. It left a horrible long jagged scar that was quite thick as well. There is only about a 2 inch or less area that is visible that is still slightly red. But amazingly that ugly thing is finally nearly gone after two decades.
I am not sure how a juice fast would work, but give your body whole healthy raw food or fresh juices ( not the stuff on the shelves) and the body will repair itself. Same is true with fasting as long as it is a pure fast ( no diet sodas, pills, mints etc, sugarless gum).