Plaster, p2
Preparation
For plaster application on straw bales, three coats of plaster are generally recommended: the scratch coat, so named because it is scratched, or scarified so as to provided keying for the next coat. The second coat is called the brown coat. Both of these plaster applications should be 1/2 to 3/4 of an inch thick. The scratch coat is mixed to a 1:2 ratio (1 part lime to 2 parts sharp, well graded sand). The brown coat is a weaker mix, the ratio being 1:2.5. The third and final coat is the finish coat. It contains any pigment required to achieve the desired wall color, and is only 1/8th inch thick. It also contains only very fine grades of sand.
Tape and plastic sheeting protect the doors, windows, and ceiling from overspray, while construction paper and roofing felt cover the floor.
Before the plaster can be sprayed onto the walls a few preparations must be made. Taping plastic sheeting over the doors, windows, window sills, electrical outlets, and ceiling borders will prevent an overspray mess. A layer of construction paper on the floor held back from the wall the thickness of all three layers of plaster, and then covered with roofing felt will protect the floor.
The evening prior to the day of plastering, the bales should be sprayed down with water several times. The next morning they should be sprayed again before plastering begins. The bales do not have to be dripping wet, but they should contain enough moisture so as no to wick the moisture out of the plaster once it is applied. Damp or even wet to the touch is OK, but not dripping.