1950s to 1960 house with rock lathe (dry wall) covered with a grey brown coat (wire mesh in corners) and then a thin white plaster coat. Was used as a doctors office and we are putting back to the original floorplan (adding a laundry room). Plaster of Paris should work as the top skim coat, but does Any one know the formula of the sandy grey brown coat???
The brown coat s a very weak mix of mortar. Back in the day it may have been a specialized mix for stickiness, so it stuck to the rock lathe. You could use as an alternative a ready mixed mortar for stone veneer. All or at least many repairs I seen in homes with rock lath and plaster, were made by using metal lath, scratch coat, base coat, then white coat.