Working with Castable Refractory is not like working with regular cement mix. If you add too much water when working with cement, you just add more dry cement mix. If you didn’t add enough water, you just spray more in. Easy Peasy.
Castable Refactory on the other hand requires an exact amount of water to be mixed per bag to ensure the Castable cures properly. Too much water and the Castable will be weak. Too little water and the Castable will be too dry to pack. Both can result in possible hairline cracking.
The first tip is to follow the mixing instructions set forth by the Castable Refractory manufacturer!
The second tip is to pack the Castable Refractory slowly yet firmly. Castable is a fairly sticky and as you are packing it in the foam forms, the Castable will tend to stick to your wood dowel (the stick you are using to pack the Castable). If you are packing the Castable loosey-goosey, the Castable will stick to your dowel and leave an air pocket and/or not pack properly.
IT IS VERY, VERY, VERY IMPORTANT THAT YOU FOCUS WHILE PACKING THE CASTABLE!! MAKE SURE YOU ARE PACKING IN SMALL BATCHES AND ENSURE THAT THE CASTABLE IS PACKING FIRMLY AND THERE ARE NO AIR POCKETS.