Paper varies widely and some will have no sizing. Thicker ones are less prone to buckling with water media.
For best results, use the media as recommended by the paper manufacturer and test.
When working on these types of papers, acrylics, watercolors, and dry drawing media, a barrier is not required. However, when painting with oils on paper, an application of 3 or more coats of Acrylic Gesso is recommended to block oil absorption.
Grounds, gels, and pastes can be applied to alter the surface’s absorbency, durability, or texture.
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