There are many hundreds of Japanese papers (washi), hand made specifically for watercolour printing. Selection can be quite a long process.
The paper usually needs to be sized. However, Japanese paper gets stronger as it gets older, so sometimes sizing is unnecessary.
The size itself can take a couple of days to make. Its ingredients include Nikawa, a type of animal glue, and alum. These are ground, melted in water, and evenly applied with a large soft Dosa brush. Once the paper is dry, I try to leave it for several days to allow the size to settle in.
The paper is moistened by layering between several dampened sheets of newsprint. These are wrapped in layers of plastic that allows the moisture to spread evenly. It takes 2-6 hours for the paper to dampen ready for printing.