Jorge,
I have used Knox gelatine to size prints for salt paper. I use the formula that mixes the gelatine with the salt--I'm in process of moving and all my books are boxed, so I can't give you the exact formula, but it was listed in two different books. The formula was for small amounts to be brush coated, but I mixed enough to dip the paper in, since both the gelatine and salt are relatively cheap. I floated the paper and coated both sides, then hung it to dry. Coating both sides keeps the paper from curling. Paper was Fabriano Artistico. It worked great.
Ed