Usually, it is not necessary to treat the high quality papers that are to be used in alternative photographic processes. They are ready to use as they come out from the paper mill, and they are always sized in the production process, because otherwise they would absorb too much and colors would tend to penetrate into the fibers and disperse, like it happens with blotting paper

There are two cases, though, when paper sizing is necessary:
1)  several negatives are to be pin-registered and printed in sequence;
2)  the paper's surface is way too absorbent.
Platinum printing need not gelatin sized papers. These papers are to be used without any pre-treatment.

In the first case, a shrinkage would take place right after the first wet and dry-out cycle, making it impossible for the printer to get a perfect registered image in the following cycles.
To avoid this, it is necessary to have the paper sized in one or more cycles, wetting it in warm water and let it dry. This treatment gives the paper a good dimensional stability, though taking out the original priming, which must be put back in place afterwards.

In the second case priming the paper is enough, like in the second phase of the previous treatment.

The work to be done here is not difficult, but since it takes quite a long time it is advisable to treat as many sheets of paper as possible at the same time.

Soak the paper sheets in cold tap water for about half an hour; in the meantime heat some water to about 60°C.
Pour the hot water in a tray and immerse the sheets one at a time, leaving them inside for about 10-15 minutes with constant agitation.
To speed up the process you can add additional sheets every 5-10 minutes, then pulling the first one to the surface and so on. When the immersion time for each sheet is over, hang it at room temperature and let it dry.
The tray temperature must be regularly checked and kept at about 60 C°, adding hot water as needed.
Once the paper sheets are dried, the whole process can be repeated.

Many products can be used for sizing (natural like gelatine or starch paste, or synthetic like hydroxypropylcellulose or Aquapel ®). In these notes we will refer to gelatine because easily available.
Two steps must be taken: sizing and hardening.
With the first one we put gelatin back on paper, filling pores in the paper, while hardening makes gelatin insoluble giving permanency to the sizing.

Tools:
Gelatine
Hardening agent (formaldehyde or glyoxal , in 40% solutions with water) Agente induritore (aldeide formica o gliossale, entrambi in soluzione al 40% circa)
Anti-fermentative agent (thymol or salicylic acid or sodium azide)

Warning:

Gelatine is sold as "fish glue" for kitchen use. It is ok, if not too old, because time alters some of its properties. Gelatine is sold in sheets or pearlsl. a gelatina viene venduta anche sotto il nome di "colla di pesce"..

Formic Aldehyde, also known as formaldehyde or formalin is sold as 40% solution and well known as cancerogenic. For no reason its vapor should be inhaled.
IT IS NOT TO BE USED INDOOR, even diluted!.

Formaldehyde can be replaced for glyoxal, which is less volatile and then less dangerous. The same safety rules apply to glyoxal as well, but it can be handled with more confidence than formaldehyde

Should you want to keep the gelatine solution for some days after its preparation, you have to add some anti-fermentative agent, or the gelatine will form moulds, losing its original properties.

2.5% gelatine solution (25 grams per liter)
Weigh the necessary amount of gelatin with regard to the final volume, and soak it in cold water for about 30 minutes.
Eventually weigh the anti fermentative agent, calculating:

0.5	grams per liter in the case of	salicylic acid
0,2	"              "	thymol
0,01	"              "	sodium azide

Warm up the water to 40°C, add and dissolve the anti fermentative.
Take out the swollen gelatin from the cold water and dissolve it slowly under agitation in the hot water.
Warning:

To facilitate s the gelatine in the cold water, break any lump with a glass rod.

Do not heat the water more than 40°C as the gelatin may deteriorate.

Hardening solution
Measure 20 ml of formaldehyde or glyoxal and add them to 1000 ml of cold water.
Warning:

this operation should be done in a well ventilated area, better outdoor, as formaldehyde causes cancer.

The formaldehyde solution has indefinite shelf life, while glyoxal must be used within a month.

Paper sizing:
The paper can be primed with a brush or by immersion. The second technique takes longer and requires more work, but it ensures more uniform results.
Bring the temperature of the solution to about 40°C.
Pour the warm solution in a tray, then immerse the sheet of paper. Make sure it stays immersed for about 10 to 15 minutes turning it upside down a couple of times, paying attention in removing air bubbles which may appear on the sheet's surface.
Take out the sheet letting its down side slide on the tray's border. Lay the sheet on an horizontal flat surface until the gelatin will begin to solidify.
Gently detach it from the surface and hang it to dry.
Warning:

Mark the down side of the sheet to recognize it.

Any air bubble which should remain on the surface may cause irregular sizing.

More diluted solutions are more easily absorbed by the paper's fibers, but give a lighter priming, while more concentrated ones may give inconsistent results. We do suggest to repeat the entire process twice instead of increasing the gelatin concentration. Repeat the process BEFORE the hardening step.

The gelatin must be solid before hanging the sheet of paper, or it may begin to slide down the paper resulting in a non uniform sizing.

Hardening:
Immerse the primed paper in the hardening solution letting it rest for 10 to 15 minutes with a mild agitation.
Take the paper out the hardening bath, let the excess solution drip away and hang it to dry.
Warning:

Let the paper dry outdoor, especially if formaldehyde is used, or in a very well ventilated room.