The subject of paper came up in class the other day - questions about sizing and such - so I did a bit of research to try and supplement my experience. I learned a couple things and got some surprises too. Had I gone by manufacturer info and Internet reviews I may have rejected my favorites without trying them!
First, my favorite papers:
1. Bockingford 140lb. Cold Press (white and occasionally the tinted)
2. Saunders Waterford 140lb. Cold Press and Rough
(Both are manufactured by the same mill)
Next, papers which work ok once I get the hang of them:
5. Bee (?) a paper I was given to try.
Last, papers I don't like:
1. Any 300lb. paper (WAY too thirsty for me)
2. Most hot press surfaces (except in sketchbooks)
3. Papers with an obviously mechanical texture (most "student" papers)
Watercolor papers are sized to reduce absorbency (think painting on toilet paper) . All are internally sized; sizing mixed into the pulp before forming the sheet. Some are externally sized as well; an additional coat applied after the sheet is formed. This extra step accounts for the higher price of artist grade papers. Student grade papers don't get the extra application.
Watercolor paper can be made from wood pulp or cotton. Cotton is naturally acid free. Wood contains acidic lignin but high alpha cellulose pulp can be buffered to a neutral ph. Cotton's longer fibers supposedly make it less buckle-prone than wood fiber papers.
Watery pulp is spread onto felts to form a sheet. As the water drains and evaporates the sheet takes on the texture of the felt (sheet by sheet with handmade papers or in a machine which duplicates the process in volume). Cheaper papers are produced on a machine which doesn't mimic the felting process as well. This results in the pulp fibers assuming a more uniform grain direction. These papers buckle more than felted papers, whose fibers are more randomly distributed, equalizing their expansion and contraction a bit when wet.
So, logic would dictate the best paper is 100% cotton, externally sized and favorably reviewed by a diverse, international cross section of experienced painters on Internet sites such as WetCanvas...so much for logic...
Ironically, my first choice, Bockingford, meets NONE of those criteria! My second choice, Waterford, meets 2 out of 3.
Arches, which doesn't really thrill me, enjoys a mojo-like reputation. Brits speak highly of English made Saunders but is that national pride? Painters outside the UK seem to be Arches fans overall.
If you learn to be aware of a paper's condition (dry, wet, damp) you will have a fair idea what happens when a given consistency of paint hits it. Experienced painters get what they want on just about any paper because they can gauge its condition.
How paper reacts to water will differ between wood and cotton, the weight of the paper and the type and amount of size. All added variables - some of which are patented and proprietary.
Your style will dictate a favorite. If you rework your surface a lot or use masking fluid liberally a heavily sized, long fiber paper is indicated. Some artists don't like how Arches behaves until they've brushed water on it or soaked and stretched it. I suspect that softens or removes some of the size and gets it to a state similar to Bockingford, which is only internally sized. If you gave Bockingford the same treatment, it would probably be unsatisfactory.
Alvaro Castagnet and Joseph Zbukvic have Arches endorsements (interestingly they both used Saunders when they had to buy their own) but they can paint well on anything because they're hyper aware of the paper's condition. Both paint in ordinary sketchbooks made primarily for graphite. I've seen Zbukvic remark on video how Arches doesn't react quite how he likes but he adapts in realtime and the painting comes out.