When I removed the form i noticed several voids around some of the firebrick. In some spots the voids are approximately 1.5 inches deep. Should I try to fill and if so, what product do you recommend?
If the voids are deeper than .5", go ahead and fill them in w/ fresh refractory mortar. If it is deeper than .5", you don’t need to worry about them (as the mortar is still 4" thick).
If you fill any voids less than .5", you run the risk of the new mortar not adhering to the oven - it would be like a patch… and patches can possibly fall out.
If a patch falls out into your food - it can result in a very crunchy pizza.