I found this article online from an Italian magazine, which explains the gelain sizing for oilprint (which has the same basis). The gelatin layer should be thick and dense.

- Soak 6g of gelatin in cold water. Let swell for about 20 minutes until it is soft.
- Heat 100ml of water at ~60C and pour the [I suppose, drained] gelatin a little at a time, stirring with a spoon.
- Soak the paper in hot water for a few minutes, then lay it on a flat, levelled surface (like a glass plate), after having removed the excess water.
- Pour the hot gelatin on the sheet with a spoon, spreading it with the bottom of the spoon. Avoid spilling outside the sheet borders.
- Remove any air bubbles by poking them with a pin and leave the sheet flat until the gelatin hardens. This may be several hours.
- test the surface on a corner of the sheet, touching it. The gelatin should be soft but firm. Hang the sheet to dry, for up to 24 hrs. if needed.

I have a doubt: does the paper need to be acidified?