When you use water-based media, achieving amazing results relies almost as much on the paper surface as on the quality of the colour used to create artwork. Water colour paper must provide a suitable surface for painting according to three key factors.
If you intend using a lot of water in your water colour painting, then it is important to stretch your paper before starting to avoid cockling - this is where the sheet wrinkles and forms ridges that are almost impossible...
The beauty of pastel is best exploited by using a paper with some degree of texture; not only because the pastel has a surface to adhere to but because the visual effects are more varied than when using smooth sheets.
It is not advisable to apply any Winsor & Newton oil colours onto paper that has not been primed. Ideally, it is recommended that you use paper that has been prepared specifically for oil colour such as Winton Oil Paper.