I have seen this happen many times, a beginner learner comes to the class very disappointed because the paper has warped or buckled. So why does this happen and what can you do about it?
It usually happens with watercolour painting but it can also happen with gouache or acrylics if they are well diluted with water. It is because of the high content of water and the absorbent abilities of the paper. Most papers can not cope with such watery mixes and will buckle and warp when wet and dry that way. Paper that is less than 200gsm is going to buckle with watercolour so there are two solutions:
Buy thicker paper
Painting requires much thicker paper than drawing. Any papers that are designed for drawing – like the paper in your drawing sketchpad isn’t really designed for painting. Most painting requires thick paper of at least 300gsm if you are working in a very diluted way with water. Even if you work with fairly thick paint you will need a robust paper of 250-300gsm for the best results.
Make sure you are buying a sketchpad that is suitable for paint if you wish to draw and paint in it. Thicker paper will be more expensive, some watercolour paper is £5 a sheet but it will solve this problem. As a beginner it is understandable if you don’t want to be spending so much on paper. Sketchpads are a more economic way of buying thicker paper rather than loose sheets.
Stretch your paper
You could stretch your paper, this is a simple process and a cheaper solution. I have published a post about how to do it see Why Do I Need to Stretch Paper? for more advice.
Is all Lost?
Lets say that you have already produced a piece of art you like and it has wrinkled, is there anything that can be done now? You could try putting some heavy books on top of your work, it won’t do miracles but it may help a little in flattening it out.
You could frame the artwork and if you have allowed enough space around the edges use masking tape or brown gum tape to fix it into place.
Also see my post What is Watercolour?