It is not recommended to fill expansion joints when coating your floor.
Filling joints with a hard-curing epoxy or cementious patching material will counteract the purpose behind these joints and eliminate the slabs ability to shift. This will inevitably lead to cracking of the concrete. Joints should likewise not be filled with caulks or silicones (even if they state they are paintable), as some of our two-component systems will cure too rigidly to be applied over a flexible material.
When applying a coating to the floor, take care not to pour product near the expansion/control joints or to push heavy piles of material towards the joints with the roller. You may use a roller or a chip brush to apply a light coat of material onto the visible/exposed areas of the joint.