A.Much depends upon the cause of the problem. If the top (white) coat is delaminating from the brown coat, a latex bonding agent will be needed to make the new surface adhere. A lime finish plaster should be used. If the area to be patched is small, you can use patching plaster, which is almost pure gypsum. Of course, if the flaking and crumbling is caused by water damage, the water source must be removed before any patching will hold. For more complete instructions on plaster repair, I suggest Old House Restoration Pamphlet #1, Plaster , available for $1.50 postpaid from the Building Research Council, One East St. Mary’s Road, Champaign, IL 61820.