I am doing a plaster patch over a sandstone wall in the 1897 apartment this morning.
Step 1 I paint the patch area with dap concrete bonding agent mixed with water. A couple of table spoons to a gallon.
I also put a few table spoons in the water I am about to mix my Diamond plaster in.
I am using Diamond Base Coat. A USG product.
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From @andre-ager