Most good quality full sheets of watercolor paper come with a shipping sizing on them. The sizing is meant to protect the paper from fingerprints and during shipping, even if they are in multi-sheet packs. Note: pads and blocks do not have this sizing.
The problem is many artists do not know that it is there. For years, artists have used the paper direct from the store, not realizing that they were having to paint through the shipping sizing to get to the paper. This just shows how adaptable watercolorists are and how anyone can make a product work for them.
Removing the Sizing
To use the paper as the manufacturer recommends, the shipping sizing should be removed. I soak the paper for just a few seconds in lukewarm water. The shipping sizing feels a bit like talcum powder and melts very quickly; therefore, you do not want to soak the paper very long. You certainly should not soak it long enough that the water starts to get absorbed into the fibers of the paper.
I dry the paper by attaching it by the deckled edge to a shower rod with some alligator clips attached by twist ties. I let it dry overnight. My experience is that is dries as flat as when it was purchased. If it curls, that means that it was soaked too long.
Making the Best Choice at the Art Store
While the paper has this shipping sizing to protect it in stores, do not take that at face value. You never know how paper has been handled or abused in a retail setting. I highly recommend buying paper in three packs, five packs or whatever numbers you can get the paper that is still sealed in its shipping plastic sealed wrap. You will also find that buying paper in the multiple sheet packs greatly lowers the per-sheet price. If you must purchase a lone sheet, try to get one out of a freshly opened package.
For those looking for an environmentally friendly option, Fabriano is in the process of changing its sizing to a vegetable sizing. I do not think any of that paper is in general distribution yet in the US — the US distributor is just beginning to receive some of the 140-lb. paper. But the new finish should be on the paper that is shipped to the stores by the end of 2020.