Most common papers aren't very suitable for watercolor paintings. Printer paper is far too thin for watercolor. It will turn translucent when you apply water to it. Since it doesn't contain any sizing, it will absorb the water too quickly and buckle.

What can I use if I don't have watercolor paper?

Cotton is the best material to use for watercolor paper because it is very absorbent and strong. Its strength permits artists to do a variety of watercolor techniques without worrying that the paper will rip or pill. Cotton watercolor paper is also acid-free, meaning it won't yellow over time.

What paper can you use for watercolor?

In general, watercolour papers are made from one of two materials; cotton or wood pulp. 100% cotton papers are professional quality, and are considered to offer the very best painting surface. Cotton gives incomparable stability and ensures that you work will stand the test of time.