ITC-100HT Ceramic Coating for Refractories

[Bverysharp]

You might like to try mixing fire clay with zircon flour & 20 grit fire brick grog. I experimented with quantities, but treated it like a concrete mix 1:1:3. The Zircon has a melting temperature around 3600C. I coated the inside of my gas-fired aluminium melting furnace with this, and it has held up far better than the portland cement & fireclay mix that I used when I initially made the furnace. The Zircon flour costed me about $12 for 5 Kg, but you don't need a lot.
You should be able to get it from pottery suppliers.