Fast - Hard 25 Si and Fast - Light 10 Si B72 Retouching Gels are recommended to use for underpainting. They can be used not just for painting textures, as in the case of Fast - Hard 25 Si gel, but also diluted and mixed with a normal solution of retouching/inpainting medium of Paraloid B72 for underpainting smooth surfaces. They are especially useful for the underpainting of large areas that will require extensive retouching.
The content of fumed silica makes paint made with Fast - Hard 25 Si Fast - Light 10 Si gels more resistant to dissolving during further retouching on its surface.
Paraloid is the trademark of Rohm & Haas