If it toughens and increases reflection of heat back into the chimney as stated other than cost it looks like an ideal addition to RMH builds using the ceramic fiber insulation.
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Sound like the perfect solution to prevent air born fibres, I use a similar product with similar claims, it works OK but does crack up after a few fires, however still stays in place.
I haven't used that product. This was what I used to coat the internals of my batch box core. I don't have experience using it on ceramic fiber insulation.
There is a very comprehensive thread at this link at Donkey's forum about refractory coatings which is what lead me to use the product that I chose.