Only use the rigidizer on the final layer. You mention "castable" - which are you using? Kast-o-Lite or a different castable? My understanding is koawool next to metal (1 or 2 layers, doesn't matter), with Kast-o-Lite (or other castable) that builds up perhaps 1/4" or so, maybe even less? Then a final coat of ITC-100 that's a thin layer - paint coast thin.
You gonna LOVE the new forge.
Sry I forgot to address this comment.
Yes 2 layers of kaowool up against the metal, then rigidize just the inner layer, then the castable (MIZZOU CASTABLE PLUS), and then the ITC-100.
So only a 1/4" of castable? I thought possibly more? I was certainly thinking the floor to be over 1/2"? Do you use any type of kiln shelf?
I can't wait to fire it up though I'm probably over a week out!