I'm on something of a watercolour jag at the moment - working on my third this week, and will post pictures next time.
Watercolour paper comes in various weights, one of the most common being 140lbs, otherwise known as 300gsm. This gets confusing to someone like me, who struggles with figures and symbols. In short, this paper is one of the thinner ones (there are even thinner). This can be - and invariably is - a problem if you use a fair amount of water with the paint (which by definition, you do). The thinner papers cockle - they corrugate horribly when you're painting and only gradually lie back down as they dry. So if you're trying to paint a straight horizon line (note to all learner painters: horizons must be level - water doesn't flow uphill) it's the devil's own job to get it right: you can't really see it properly until the paper dries out and goes flat.
To counteract this problem - I wish I could say entirely defeat it, but that's something of a matter of chance - many people stretch their papers, soak them in water (and that really means SOAK) lay them down on a board, and stick them to the board with gum-strip. There are video demonstrations of this on YouTube.
Apologies to all who know this already.
When the paper has dried out, the idea is that the gummed paper will hold it in place as it shrinks, thus giving you a flat surface on which to paint which won't cockle. But it doesn't always work: the gummed paper lifts, or the paper wasn't wet enough so that all the fibres were saturated, and you still get cockles; but should get fewer of them.
Stretching paper, however you do it, is however a bit of a faff .... and as soon as I reach the last sheet of my present batch of paper, I'm going to buy some heavier weight paper (300lbs or so) which is more expensive, but doesn't cockle and doesn't need stretching. What I want to know is - WHY do so many artists' suppliers offer only the lighter weight papers? And why are they so conservative in the range of papers offered? There are many more than the standard Bockingford and The Langton (nothing wrong with those, mind). Arches, Fabriano Artistico, Saunders, Schoellershammer, Hahnemuehle, Two Rivers, and more. I can understand why art shops don't carry all the ranges available, but could the bigger companies who trade online not offer more ranges and weights, so that beginners don't get lumbered with the most difficult papers to paint on?