Do you always use acrylics for underpainting?

Yes, my preferred medium for my block-in is acrylics due to the fast drying time, so that I can quickly move on to the details in oils. I actually enjoy blocking-in in oils, but for my busy schedule I have chosen acrylics for block-in. It is entirely up to you if you prefer oils for your block-in and if you have the time to wait for the oils to dry. Winsor and Newton Alkyd Fast Drying Oil paint can also speed up the process of using oils for a block in.