Cum crust is available in two forms. First is the cum from a penis that dries on a vagina after sex.
The second is when a female gets sexually excited and excretes her lubricant onto her panties where it dries and becomes crusty.
After making love both male and female bodily fluids, cum,soak the vaginal area and the penis. If one does not wash after sex and allows this to dry a thin white layer of cum crust will form on the head of the penis and the vagina resulting in cum crust.