Batch Rocket core of top-coated insulated fire brick?

I am building a batch box rocket with a modification - and I was hoping more experienced minds could respond if this change is a good idea.  
I am using Peter Vandenburg's design as shown on the batchrocket.eu site for the basic bire brick core. My proposal is to use insulated fire brick instead of hard brick, but then to top coat it with refractory cement.  

So I'd build the chamber 1 inch "too large" with the insulated brick, and then put a 1/2 of refactory cement on all the way around on the interior surfaces.

My goals are:  Use the softer brick to be easier to work with.  Make the burn chamber interior is durable against abrasion. Have a burn chamber that gets hot faster (since it doesn't leak away the heat).  Have cooler temps on the exterior walls of the batch box. Avoid having to cast from a mold.
My concerns are:  will the refractory cement bond well enough to the brick or will it fall hopeless off after a few burns and some heat stress?

I'd sure be grateful for any advice or insight folks have about this method.  

Thanks.

Hi Jonathan,

I do not know what the refractory cement does in a thin layer either, but you may want to build a test rocket and try it out on a small scale before going into production.  ;-))

One ideas that I have had in regards to some of my own projects, but not yet been in a position to test it is to make a clay slip with a slightly diluted water glass mixture  Then layer 2 or 3 coats onto the surface to protect.  Once it is fired, the surface should get very very hard and note I say should.  I am an engineer, but this is NOT my primary area of expertise!  I am learning and having fun in the process, but I still have a good long ways to go in my learning process.

Best wishes on your rocket endeavors.