Help With Identifying Thin White Coat Laid on Coarse Lime PLaster

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Hi All

Would be grateful for any help. Specifically what is this 1mm thin very white final coat that's been laid on v coarse lime plaster (hairy with pebbles <10mm).
Is this usual for a vict 1900 terrace? The coarse plaster looks more like dirt than plaster and is v weak and easily dissolves in white vinegar. I'm stripping wallpaper and need to figure out if i need to go back to brick and how.

Thanks in advance