I would recommend magic sculpt or some other product that is? 1. water soluble. 2. Sandable and 3. Non Shrinking. (I will echo Scotts recommendations also)
For flat gaps I mix up just over what I will need to fill the gap, apply with water, let set for about a minute, shave with some flat surface like the edge of a slotta base, check and fix any fissures, let dry and finally sand with a small square of fine grit.