When all pages are ready, then I find most of them are slightly wavy.
And you know the wavey papers are not good to be scanned,
they tend to make uneven shade in the scanning result.
An easy treatment for wavy watercolor art,
that won't harm the art on the paper is simply with taking
a slightly damp sponge and wet the BACK of the paper just so it is moist.
Once the paper is damp, put it between blotter paper and weights
and flatten it that way. I am using a heavy frying pan, here..:D
Trust me, it works and doesn't have any effect on the art on the front.