Good paper is the most important of all watercolor supplies.  Don't make the mistake of starting with
"cheaper-inexpensive-student-grade" paper with the intention of upgrading to better paper when you improve your skills. Learning to
paint on bad paper creates bad habits. Good paper is worth the investment. Watercolor paper is available in three different textured
surfaces, three different weights, and several types of packaging (depending on manufacturers.) Look for archival paper that is acid free
with a neutral pH balance. 100% cotton rag is a better quality than buffered papers.
Commonly there are three surface (types) of paper available:

Rough - A rough paper has a heavy “tooth” which can be used to get a more textured
or dramatic effect.

Cold Pressed (CP) or (NOT) -- A paper 'not' hot pressed with a slightly rougher surface.
CP is the easiest to use and is best for the beginner. It has a texture that allows it the
stay wet longer and makes it a favorite to work with when learning to paint watercolors.

Hot Pressed (HP) -- A smooth paper which is manufactured by a method where heat
and pressure are used as suggested in its name. Great for details and sharp edges.

watercolor papers are being introduced that have two surfaces. One side is hot
pressed (smooth) and the other cold pressed (semi-rough). I have not had the pleasure
of trying these yet.
Online resources for purchasing WATERCOLOR PAPER
Watercolor Supplies and Materials - Papers
The surface of the watercolor papers range from very slick and smooth to very textured and rough.  Each surface has
its own personality and handling characteristics. Many award winning watercolor artists select their preferred paper
depending the surface that best fits their style of painting which may differ greatly from the choices of their peers.
Paper Surface
Paper quality and durability varies by manufacturers. 140# CP paper made by Strathmore or Canson is different than
140# CP by Arches or Lana or Winsor & Newton. Each brand has it's special purposes but they ARE NOT equal in
quality and uses

Watercolor paper is available in various sizes in:
  •  Single Sheets (22 x 30) that can be cut into various sizes
  •  Spiral Bound Tablets/ Pads
  •  Watercolor Pads
  •  Watercolor Blocks
The paper's sizing will also effect the papers surface. Sizing is added to the paper pulp (internal sizing) when the paper
is made and is sometimes sprayed on the surface (external sizing), sometimes both. Sizing is a gelatin based substance
that helps control the paper's absorption and durability and varies by manufacturer. The best papers are both internally
and externally sized.
Paper Weights

The most common weights for watercolor papers are:
  •  90 lb light weight  - student grade this paper is easily damaged when scrubbed
    and buckles easily when wet (requires stretching)
  • 140 lb medium weight - good quality - artist grade durable, holds up to vigorous
    scrubbing, will buckle when wet if constrained without being stretched.
  • 300 lb heavy weight - good quality - artist grade durable, thick, doesn't require
For beginners I recommend:
Arches 140# CP -full size single sheets (22 x 30) divided into 4 pieces (also called quarter sheets)
or 8 pieces (eighth sheets) for learning the fundamentals and basic exercises and studies.
Tearing the paper to size yourself will save you some money. Stick with one brand of paper while you
are learning. Test additional paper brands when you have a good foundation of basic skills.

My personal are favorites are:
Arches 140# CP sheets and Arches 300# CP
Occasionally, I will use Arches 140# or 300# Rough for added texture

I've experimented with Jack Richeson 140# CP and Fluid 140# CP watercolor paper and found both