Some points on sizing ... (entry by Mary Krieger - in response to some questions on pigment absorption ...)
Here's my two cents on the pigment/sizing questions.
I find it helpful to remember that all paper is a web of tiny plant fibers held together both by their physical entanglement and electrostatic attraction (at least that's what I remember the instructor saying). The tiny spaces between the fibres allow paper to flex and compress.
Sizing is gluey and coats each individual fibre. It is actually quite difficult to wash out once it is in. It alters the ability of the individual fibres to absorb water (like fabric softener on your towels).
Dry pigment applied to a paper surface adheres like the paper fibres by being caught in the microscopically uneven surface of the paper as well as that static attraction (like dust and cat hair).
Pigment suspended in water will penetrate more deeply into the microscopic structure of the paper. As the fibres soak up the water, the pigment is both caught in the holes between the fibres and stains the fibres themselves. Some of the pigment between the fibres can be rinsed away with sufficient water but the stain can only be bleached away (like grape Kool-Aid).
Sizing will slow down the rate at which water is absorbed by the paper fibres. Watercolor paper is very heavily sized. This allows the painter to control how the pigment is absorbed by the paper primarily by the amount of water in the paint. The fibres are so heavily sized that they absorb the water very slowly and do not much affect how the pigment is absorbed by the paper.
If the paper is already damp - that is the fibres themselves are moist but there is little or no water film in between - then I would guess that the pigment would move more slowly but would be absorbed by the paper more evenly.
If moisture promotes the absorption of the pigment into the paper - and rice paste and size slow it down, then to get an even and repeatable penetration of the pigment into the paper you have to balance the amount of sizing and moisture in the paper with the amount of rice paste and water suspended pigment on the block.
Prints on paper that is correctly sized and dampened will not bleed sideways or transfer color. That's the amazing trick of the Japanese printing technology.