I've just about given up plastering/sealing around my new braai. I've tried high head product that you mix in with cement and now high heat sealant. The high heat sealant has exposed what the real problem is. Excuse the mess, it's a mixture of cement and ash from test fires.
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When the braai gets hot the hole thing pops forward by a couple of mm, so whatever is stuck to the sides of the braai then comes off the wall with it. It's not the heat so much as the movement of the braai.
It's probably because my builder screwed up the installation of the braai, which wouldn't be a surprise.
I think my best bet here is going to be to get a flat aluminium strip and stick it against the wall all around the face of the braai, but not touching the stainless steel of the braai itself.