Thistle undercoat plaster; manufacturer says if necessary to achieve the
required thickness, build up basecoat in 11mm stages.

If applying the base coat in two layers, at what stage should the second
layer be applied? Is it when the first layer has just become firm but not
yet gone hard, i.e. when the first layer is at the same stage you would let
a base coat get to if you were then going to apply a top/skim coat?