Some guidelines (not rules) for selecting the right texture and weight of paper.
Watercolor paper can be classified by texture and by weight.
Texture refers to the smoothness of the paper: hot pressed (smooth), cold pressed (slightly bumpy; also known as “Not” as in Not hot pressed), and rough (highly textured; often labeled as “R”)
Weight is the weight of 500 sheets (one ream) of the paper: 90 lb. (student grade), 140 lb., 300 lb. Heavier papers are stouter and thicker, and can take more water without buckling.
Here are some general guidelines or suggestions (not rules) for selecting the right texture and weight for your task.
Landscape, still life, flowers, abstracts, loose figures
Texture: Cold pressed
Weight: Will you use a ton of water and lots of layers? Yes: 300 lb. No: 140 lb.
Line and wash (also called pen and wash; use a pen to sketch, then add some watercolor to enhance the sketch)
Texture: Hot pressed. The bumpy nature of cold pressed and rough makes the initial pen sketch difficult.
Weight: Will you use a lot of watercolor on it? Yes: 140 lb. No: 90 lb. will work if it’s just a splash of watercolor here and there.
Portraits, photorealism, very detailed painting
Texture: Hot pressed
Weight: Will you use a lot of layers? Yes: 300 lb. No: 140 lb.