Which white should be used for underpainting?

Linseed oil is the preferred medium for underpainting in oil painting. As oil paints dry, the paint film undergoes various dimensional changes, increasing and decreasing in weight due to different chemical reactions. These movements can lead to cracking in the layers applied above.

Linseed oil dries quickly and undergoes fewer dimensional changes compared to semi drying oils like safflower and poppy oil, making it more suitable for underpainting.

While a safflower oil based white is perfectly appropriate for normal applications and mixing, it is not suitable for underpainting.

Artists' Oil Colour Underpainting White

Winsor & Newton’s Underpainting White is formulated with linseed oil instead of safflower oil. This allows it to dry much faster than other whites, such as Titanium White, which helps prevent cracking that can occur when using slower-drying whites as a base coat.