One slightly daunting aspect of watercolour painting for a beginner is the cost of the paper. Even at competitive prices, a quarter imperial (11" x 15") piece of quality watercolour paper will cost around a pound sterling (UK), so a serious mistake hurts a little in the financial department. Risk can be lowered by dividing the paper into eighth and sixteenth sheets, but this is not conducive to a looser painting style using larger brushes.
I have been a customer of Ken Bromley Art Supplies for a few years and it didn't take me too long to notice that they sell a 50-sheet quarter imperial pack of practice paper, at around 30 pence a sheet. The paper is Fabriano 280 gsm watercolour paper in a Not surface, which works out to 134 lb thickness. This is about a third of the price of popular 140 lb watercolour papers and is worth considering as a practice surface if you are not too worried about the outcome. Even the reasonably-priced Bockingford paper still works out to be more expensive in this quantity.
One mystery -- and what attracted me to the paper in the first place -- is the cotton content? Going by the Ken Bromley catalogue at the time, it appeared to be a full cotton paper. But I think this referred to a paper that is no longer sold. A couple of internet sources suggest that there might be a 25 percent cotton content, but I'm still not sure!
I duly ordered a pack and it soon turned up in the post thanks to Ken Bromley's usual efficient and prompt service. I chose to divide up some of the sheets into halves and quarters. I used the sixteenth (imperial) sheets for experimentation and testing, as well as a few finished paintings. The larger eighth sheets were used for paintings only, including a test to see how it behaved when stretched.
The first thing that struck me was the very regular pattern left on each side by the paper making process. It looks as though a wire mesh (screen) was used on both front and back, unlike the woollen felt normally used on the front of other papers. It is therefore very difficult to determine which side of the paper is the front, and which side is the back. (It probably doesn't matter very much!) The texture is also why Jacksons Art doesn't recommend this paper for finished works.
Notwithstanding the regular texture, how did it respond to paint? Well, as someone who is still very much at the learning stage, I found that it was a challenging surface. It did not appear to be heavily sized and was more absorbent than other papers I have used. Consequently I had difficulty lifting some paints, especially staining ones. Too much water and over working caused the surface of the paper to break up. And yes, it did cockle substantially after heavy washes if it wasn't stretched.
The absorbency and reaction to water meant that achieving a plausible cloudy sky was an effort in itself! (Mine have been hit and miss -- mostly miss.) I think that pre-wetting, then a heavy wash followed by quick work with absorbent material may yield the best results.
Perhaps I was lucky with my method, but the paper did seem to behave better after it had been stretched. (I used standard brown tape on a plywood board.) I felt more confident using larger amounts of water and wetter washes, and even though the paper would still lightly cockle, it would always return to its stretched state once dry. This seems to validate the recommendation to stretch paper of 140 lb weight and lighter.
This practice paper definitely fills a niche, but may be best for painters already confident with their technique and wanting to get some brush mileage behind them. I would still recommend something like Bockingford for beginner and inexperienced painters, as it is a harder wearing surface that stands up better to repeated working.
