How was Wayne Thiebaud able to achieve such thick, smooth effects for his paintings?

Wayne Thiebaud is famous for his luminous colors and exciting compositions of ordinary objects. But he was also a master at producing thick, smooth paint coverage. How did he do this?

Most paint mediums that I'm aware of (Galkyd or Liquin, for example) will produce smooth effects but the paint coverage will look a bit thin. Alternatively, you can apply paint thickly (with no added medium) but the resulting surface won't be smooth.

Ideas?

https://preview.redd.it/qxqv26s61scg1.png?width=900&format=png&auto=webp&s=83c7a38c26952ff7b8d2d201e8a3a0efeddc9bd1