Last post I touched on the role of sizing in watercolor paper and it's effect on the paper's behavior when wet. I've puzzled it some more and come up with this. I'm Using Arches and Bockingford as comparisons - there are too many papers to comment on all of them and these are popular examples of two basic types. There are no really awful papers in my opinion (though Strathmore Aquarius comes close). Just papers that wouldn't match your profile on ePaperHarmony.com. The purpose of this post is to shed a bit of light on why each might perform as it does, in the hopes you can experiment and find a suitable match...
1. Is the size IN the paper, ON the paper or BOTH?
That corresponds to, respectively, internal sizing, surface sizing or the two together. Bockingford is an internally sized paper, and this is typical of "student" papers, though for reasons below I find it outperforms them all and most "artist grade" papers too. Arches is both internally and surface sized which is typical of artist grade papers.
2. How much size is used on the paper? Here I have to guess and go by feel. I'm going to say Bockingford feels pretty heavily sized and Arches uses less internal sizing because it gets the additional surface sizing (possibly there's a limit to how much sizing is desirable?..I don't know and the manufacturers are understandably mute on their process). Bottom line, one has more size close to the surface and the other has it all packed into the paper. The behavior is noticeable as it should be.
3. What is the sizing made of? Arches uses an animal gelatin (in either or both sizings...I'd say both) and Bockingford uses something else. Possibly the synthetic gelatin called Aquapel? Bottom line, they paint differently.
4. How easily does it soften when wetted? The Arches size seems to loosen up faster than the Bockingford. Perhaps because your initial paint application is hitting size alone?
5. How long does it stay wet before it hardens again? Arches, once loosened, seems to stay moist longer. Possibly that's because Bockingford is a wood fiber paper and Arches is cotton which should be naturally more absorbent. This and the point above are probably why Arches was the paper of choice for the American School of watercolorists from 1930-1970. Their working method was rather specific.
6. Does surface sizing sink in? Seems so, especially if your brushwork is vigorous.
7. Is some of it removed by the brush during the painting process? Seems like it does. On both papers colors look "furrier" in heavily worked areas. Either that or water and brushwork push it into adjacent areas. This brings up another issue: does your painting style redistribute sizing unequally?
Final observations: The whole trick to watercolor is knowing what the condition of your paper is, so that when you bring a particular consistency of paint to it, you can make an educated guess as to the result. All the above factors affect the condition of the surface and need to be noticed from one brand to the next so you know WHEN to go into the paper with the desired paint consistency. Your frog may be a prince...but they all need to be kissed differently.